Variant #2

Monopoly + Skill Tree = !

The second variant, which builds up from the first, pushes the same concept further.
It combines the mechanics of monopoly with skill tree mechanics.

Typically, the skill tree only directly affects single player. However, as the mobile games increasingly emphasize the social connection between players, a collaborative online skill tree will enforce this connection.

The possible implementations:  

• players share a same skill tree like the monopoly players share the same game board. The basic rules of this skill tree are similar to the rules of monopoly. 
• Once players gain certain coins/points from the game, they can enter the skill tree, use their coins/points and roll the dices to move. (Coins can be obtained from in-app purchased)
• Different power ups are represented similarly as the different properties in monopoly. 
• Players can invest their coins/points to gain the power up if they land on the power up spaces. If they land on the power ups they already gained, they can upgrade it with more coins/points. 
• However, the acquisition of power ups is not permanent, players could lose them similar to how players lose their properties in monopoly.
• On the other hand, gaining certain combinations of power-ups, players will gain rewards, including, but not limited to trophy, ability boost, coins/points reward or special costumes. 
• The chance cards will give players random ability boost, bonus costumes, and penalties. 
• The jail will cause the player temporarily lose partial of their power-ups until they finish playing certain number of games. (This can be instantly removed by in-app purchased)

Advantages of this variant:

1. The interactive skill tree enhances the social connection between players.
2. Replace the heavy strategy part with interesting and relaxed chance mechanics. This increases the replayability, and enjoyment. 
3. (This also usually indicates more profits.)

Problem:

However, more mechanics may need to be constructed to balance the game when certain players dominate the skill tree (like what usually happens in monopoly).

Variant #1

Skill Tree Maze

Although skill tree mechanics was widely used and developed in PC and Console games, its application in endless casual mobile games is still in an infantile stage.
Therefore, I would like to explore two possible variants for this kind.

Keep in mind that while designing game mechanics for endless casual game, we should provide players a fun, accessible, relaxed and forgiving game environment.
On the other hand, the power-ups from the skill trees should be only served as spices without affecting the main game mechanics too much.

Although using a traditional predesigned skill tree will increase engagement in the beginning, it usually feed energy but couldn’t assure the replayability.

As a consequence, the first variation introduces the concept of skill tree maze.
Instead of a predesigned and fixed skill tree, the paths and skills in the skill tree maze are randomized.

• All the power-ups are randomly distributed in the skill tree. 
• Player can only see those skill(s) directly connected to the origin point or any activated skills. 
• Starting from the centre of the maze, player invests the skill points/coins gained from the game and chooses whatever paths he/she feels right.  

That way, the skill tree itself becomes an interesting mini puzzle game.
The chance and unknown mechanics scales down the unnecessary strategy elements for casual games, and increase the playability and enjoyment.

Conceptually

Skill tree is the mix of progressive mechanics and emergence mechanics.

As Progressive Mechanics

It is commonly believe that skill tree is a handy tool for adjusting in-game balance by game designers.

According to Wikipedia:

Designer can use it to delay player’s acquisition so they are not overwhelmed at the start. It is great for easing in new gameplay dynamics by starting players with scaled down versions, which can be fully unlocked over time. Therefore, designer can control the game flow in an enjoyable way.

Berry gave an excellent explanation of the use of skill tree mechanics in the game (2013):

There's no better way to educate your player in what they can (eventually) do in your game than by having a skill tree that they'll be looking for time and time again thinking about where they want to advance their character. This personalization also feeds a common need for self-expression in play that games often lack, and having a potent and varied talent system allows you to turn your single game's worth of content into an endless sandbox of gameplay discovery.

As Emergence Mechanics

Conversely, skill tree is also emergence mechanics. It has the ability to give player wide possibility space.  Skill tree offers an engaging gameplay in which players’ decisions on how they allocate points into their skill trees will affect their overall gaming experience as they play on. Without it, player will gain a monotonous experience because of the feeling of controlled by the progressive mechanics.

Again Berry provided a good example (2013):

A relatively simple game with the right talent tree can be played indefinitely. If you died in a game that lacks progression aspects it was simply Game Over, but now at least you get a little bit of progress towards that next cool talent that can help you overcome the obstacle that killed you.


Finally, although there are no set rules for designing a skill tree, by its nature, skills are usually arranged into arrays that start small, and then grow into a more complicated form.
These skills are usually organized into different categories by using “Nodes” (which is shown in the image below)

(image from article Let’s Spec Into Talent Trees: A Primer for Game Designers)

While designing skill trees, the design goals, the target audience demand should be our guidelines.
These are the core concepts we as designers should always keep in mind when designing a skill tree that can best serve the game.
     

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"Let’s Spec Into Talent Trees: A Primer for Game Designers". Game Development at Tits plus. Retrieved 01 Feb 2015.

Paradigmatic & Important Variants

After Diablo 2, Skill Tree mechanics has further developed and integrated into different genres other than RPG.

Today, let’s see some of the most influential and innovative skill trees in the modern video games. 

Path of Exile

One of the innovations of Path of Exile is its skill tree system. 

Influenced by eastern RPG’s Sphere Grid Skill Tree, Path of Exile provides a massive scale and beautiful Skill Tree system as we can see from the image below. Its Skill Tree system not only expanded the depth of the game, but also proved that video game can be art.

It is worth to mention that all characters in the game share the same Skill tree. Different characters start from different locations of the Skill Tree and different classes locations in relation to each other defines classes and sub-classes. This is smart because the designers only need to design and maintain one generic skill tree for every character. No doubt, this is one of the most successful variants of Skill Tree mechanics in ARPG genre. 

 Final Fantasy X

As the inspiration of Path of Exile, Final Fantasy X has the similar massive Skill Tree system. According to Yoshinori Kitase, the designer of the game, “the purpose behind the Sphere Grid is to give players an interactive means of increasing theirs

characters' attributes, so they can observe the development of those attributes firsthand.” (Wikia)

Torchlight 2

This is another example of a variant of Skill tree mechanics in ARPG. Nonetheless, Torchlight 2 didn’t create a massive Skill Tree system. Instead, it transferred the core skills of the original Diablo games into a small but efficient skill tree system. 

Each character has a three-tiered skill tree, which combines the active abilities and passive bonuses.  Take the red skills as the example, it is broken up into four tiers of power which is denoted as I, II, III bar. Every five talent points devoted to a skill will give the skill a passive bonus that transcends mere variable increases. This provides each skill more utility and gives the player more options. Player does not need to max level a single skill to let it be useful due to the existence of Bonuses. Nonetheless, three tires bring their own flavor of gameplay.  This innovation also provides the casual player a comfortable play experience, even they have a relative weak build. On the other hand, the hardcore player can also spend time to tune the perfect build. 

The only drawback of this compact Skill Tree its lack of long-term appeal and playability. Even so, it is an undeniable fact that it has one of the most powerful, and novelty skill tree ever conceived. 

Cursed Treasure 2

Cursed Treasure 2 is a good example of the integration of Skill Tree mechanics and tower defense genre. The game uses the Skill Tree to create a difficulty curve for the game. Player always gains right skill points after defeat, in order to progress to the next level. However, how to invest the skill points is also the challenging and interesting part of the game. 

Last but not least, the Skill Tree in this game also serves as a scalar of play complexity and progressive tutorial. Instead of receiving massive amounts of information at the same time, player starts with “just building a few simple towers and ending with all kinds of crazy upgrades, spell mechanics, and resource management the player must tend to all deeply rooted in the progression of this tree.” ( Berry, 2013).

Borderlands

http://borderlandsthegame.com/skilltrees/

Even first person shooters like Borderlands have found great success with the added depth that talent trees can bring to a game. However, the skill tree here is relatively simple. There is no close interaction between each skill. They are just imitation skill trees.

Upgrade Complete 2

The name of the game is self-explanatory. Yes, player can upgrade everything in the game including but not limited to game loading time, user interface, and scroll speed.

I don’t know if it is necessary, but definitely this is the one of the craziest and funniest variants of Skill Tree Mechanics.

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"Let’s Spec Into Talent Trees: A Primer for Game Designers"Game Development at Tutsplus. Retrieved 03 February 2015

Mechanics Historically How it Evolves --- “Skill Tree” P2

Diablo 2: the 1st tree!

It’s the time to go back to Diablo 2.

Influenced by tech trees of StarCraft, Diablo 2 integrated a deep and creative skill tree system into ARPG genre, which is also called Talent trees. 

(Tech tree from Starcraft)

(Talent tree from Diablo II)

Despite they were not perfectly balanced, a game industry was stunned by this innovation.  This first skill tree system broke with tradition where player can only level up character and upgrade equipment. Players were very keen to explore different combinations of skills. 

However, Skill tree system was abandoned in Diablo 3. Innovation of Skill Tree stops! :( 

Mechanics Historically How it Evolves --- “Skill Tree” P1

Definition 

“Perks” mechanics is usually treated as the ancestor of “Skill Tree”.  If it is true, then “Skill Tree” derives from “Perks” and develops into a fully configured system.

According to Wikipedia (2013):

A skill tree consists of a series of skills (sometimes known as perks or by other names), which can be earned by the player as he or she levels up or otherwise progresses his or her player character. These skills grant gameplay benefits to the player; for example, giving the character the ability to perform a new action, or giving a boost to one of the character's stats.


This mechanics is called as “tree” for two reasons:
1.    It is tiered
2.    It typically contains multiple paths (choices).

A tiered skill tree means player will need to gain certain level or skill points to unlock different skills (“Perks”). What’s more, player usually needs to achieve all skills in one tier before moving on to the next tier.

On the other hand, in some games, player is allowed to fulfill all the skills from the Skill Tree. However, each player can activate usually only partial of skills.

The following Skill Tree system from World of Warcraft illustrates these two concepts.

Icons in grey are inactive until player complete prerequisites for them (level up the skills to its required level).

Mechanics Historically How it Evolves --- “Perks”.

We cannot talk about “Skill Tree” without mentioning “Perks”.

According to Shamoon (2007):

"Perks" are special bonuses that video game players can add to their characters to give special abilities. The term refers to the general usage of “Perks” as an abbreviation of “prerequisite”.

As a variation of the power-up mechanics, “Perks” mechanics displaces the temporary ability boost concept and develops into an advanced concept, where power-ups are progressively unlocked and permanent through experience points.

The history of “Perks” can be dated back to 1980s.
The concept of “Perks” were firstly shown in two of the NES action RPGs:

Deadly Towers (1986)

Rygar (1987)

According to Wikipedia (2014), these two games were particularly notable for their permanent power-up mechanics, which blurred the line between the power-ups used in action-adventures and the experience points used in role-playing video games.

Even so, the term “Perks” was officially first used to describe this mechanics in 1997 role-playing video game Fallout:

Now, "Perks" has extensive application in modern video games.

For example:

• Call of Duty 4: Modern Warfare (2007)
• Metal Gear Online (2008)
• Call of Duty: Modern Warfare (2009)
• Killing Floor (2009).

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Shamoon, Evan (08/10/2007). "Call of Duty 4: Modern Warfare (XBOX 360): Impressions of guns, perks and multiplayer mode". 1UP.com. Retrieved 29 Jan 2015.

First Appearance

Diablo II

“The action role-playing game, Diablo II, released over ten years ago, is often cited as the true innovator of in-depth skill trees.” (Berry, 2013)

However before proceeding further, it is necessary to explore the evolutionary history of “Skill Tree” mechanic.

Intro

“Skill Tree” mechanics is one of the great inventions in game history.

It is also one of the very few mechanics blends the characters of simple, powerful, and beneficial to both the player and designer.

From a designer perspective, “Skill Tree” not only provides detailed and progressive explanations of the game, but also brings numerous possibilities to customize gameplay.
For a player, it enhances engagement, replayability and entertainment of the game.

As Berry stated (2013), “Skill Tree represents familiarity and the unknown, power and progression while pestering you as a designer to come up with new ways of making your game fun.”

But how does “Skill Tree” work its magic? Let’s uncover it together!

(Skill tree from league of legends)

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"Let’s Spec Into Talent Trees: A Primer for Game Designers"Game Development at Tutsplus. Retrieved 27 January  2015