Path System I: Introduction

The "Path system" isn't an attempt to rehash the "Path of Titans" concept (of which I know preciously little), but rather a new system that attempts to reorganize different sorts of content that already exist (like f.ex professions) as well as introducing new things. 

What is a Path? The idea behind the Path system is to provide the players with additional avenues along which they can develop their characters and gain new skills, abilities and powers as well as access to new sorts of content, new achievements and (vanity) perks, but all this in a strictly non-mandatory manner: one should be able to play the game and the endgame content without having to progress through any of these paths, and none of these paths (except those explictly intended for it) impact a character's strength in combat or one's aptitude to do instances. Also, some races or classes gain certain progress steps by default (simply by virtue of them already having access to these perks as a class feature); but whether every class should have access to every Path or some are restricted (for example only caster classes getting access to the Path of Portals) is still open.

A decent example that serves best for illustration would be how MoP handled everything around food and cooking. Cooking was made more detailed and extensive (split into 6 different categories), there was an entire economy revolving around the cooking ingredients, it had lots of daily quests dealing with that subject, the farm-storyline had a long quest chain and its own reputation faction (the tillers) tied to it, the players had a nice growing farm including option to collect animals to dress it up a bit, and in the end there was a payoff in the form of being able to farm either all sorts of vegetables or other crafting materials. And in case players weren’t interested in it, they could just leave it and instead rely on other people’s banquets. Essentially, the MoP cooking campaign had everything: It was diverse, entertaining, rewarding but nevertheless entirely optional. 
 

Progression of a path

• The early stages of a path should be easily available simply due to the fact that very baseline features (like riding, flying, hearthporting or profession skills) are part of the various paths and can hardly be denied to a player.
• On the other hand, every path should include desirable high-end perks that make a character special or unique. These high-end perks should be mutually exclusive at least to some extent regardless of the time invested, i.e. a single player shouldn’t be able to fly airships, get skin mutations/an alternative combat form and modify their armor all at once.
• The different “levels” of the various perks aren’t necessarily worth the same, but just show the different steps of a perk. A very powerful perk with 2 levels might be more expensive than a very basic perk with 10 or 20 levels. 
• Several path perks would require at least some progress made with other paths. 
• Given the scope of the perks gained, progressing on any given path beyond the most basic levels should take time. If a single player can easily advance on multiple paths at once and reliably complete all of them, something is going wrong. 
• Every path offers unique repeatable quests (for progression) which require a character to use the perks he unlocked. The more and more advanced perks a character has, the more difficult (and rewarding) the quests get.
•  Progress on any path should be achieved by a mixture of completing quests or entire quest chains, attaining achievements and gathering points (similar to reputation gains or artifact power), or completing missions attached to that particular tree. Some paths might additionally use entirely different ways of progressing than others, depending on its focus.

Potential synergy effects

While progressing on multiple paths simultaneously shouldn't be mandatory, some of the perks may or may not depend on the progress made in other trees. Examples:

• "Waterwalk" with mounts from the Path of Seas might depend on progress made on the Path of the Rider.
•  Craftable mounts (like a mechanic steed for engineering, a flying carpet for tailoring, a metal juggernaut for blacksmithing, an elemental for alchemy or whatever other possibility there is) should also depend on the Path of the Rider (I am sensing a pattern here).
• Riding mechanic mounts (bikes, gyrocopters, robots etc.) might also require ranks in the Path of the Operator.
•  High level in "taming the creature" might give additional options for travel forms.
• The "new skins/elements" customizing options from the path of the shapeshifter can be coupled with several other tree: naga-like elements are unlocked by progressing on the Path of the Seas, demonic elements (like Fel Orc or Eredar skins for Orcs and Draenei respectively) by progressing on the Path of Portals, more feral elements by progressing on the Path of the Body or the Path of the Beastmaster, Titan elements by progressing on the Path of the Builder etc.
•   The Path of the Warlord and the Path of the Builder would have a lot of interdependent features.
• The Path of the Builder and the Path of the Operator would also have interdependent features like a shipyard, the factory etc.
•   And so on.