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I'll start with the obvious disclaimers. This is going to be long, it cannot be helped as I'll have to describe not just the idea but also its design impacts on many systems. Second this is completely a WiP with many details unresolved. Finally, this won't be for everyone or work for every world- I've made my peace with that.
I start with an almost unholy simplicity at the core of this idea, every positive is balanced by a negative. Easy enough to say, but the application I intend is pretty deep. Let's start with attributes, every player has a fixed pool. Currently I envision 6 attributes: strength, stamina, agility, intellect, will power, and recovery, with a fixed pool of 200. These would all be assigned automatically with 10 assigned to each attribute except recovery which would contain the 150 points remaining when you start. Honestly, that sucks as a setup but the high recovery allow you some protection (you almost have to be one shot to die) and the ability to use abilities constantly (although the small pools will severly limit your options).
Maybe I should've started at creation, but now feels like a better time. At creation you'd have your standard race selection (as applicable), a host of appearance options and finally a base element. Personally, I'm not a fan of predetermined classes or the traditional trinity (More on this later). I bring this up now, because your introduction into playing the game is tied to your element. The very first "quest" is to raise the cap on 1 of the 5 attributes that started at 10 and I'm thinking I might tie that to the player's base element. The second part would to re-assign points to that attribute, unlocking a skill fragment (I'll get to this next). After this little tutorial you no longer are forced to assign points up to your cap.
I extend that core philosophy to abilities, and one-up most systems by allowing nearly total player control. This is where skill fragments come in, each is a part used to build abilities. There are many sources and fairly few restrictions for their combinations (mostly prereqs). The interface for assembling and tuning the parts would allow for tuning them (with a balance feature). So, if you were to take a fireball (just an example) and add a DoT feature it's duration would impact the base cost, cast time or strength of effect. BTW all skills require an element, even physical skills require an elemental force to initiate.
The working count is 6 elements, of which I intend to allow players access to use 3 in their builds. I'm using Earth, Wind, water, fire, light and dark. These will all exist in some portion as a local resource (BTW no area is entirely devoid of any of them). They serve as an ability modifier for abilities. The abilities have 3 modes I couldn't really cover before introducing this part, they can create, be neutral to or use up the local elemental force. This is essentially an advanced version of the field effect from Chrono Cross, and it would impact near by players and mobs alike. Create and use element would be options to add to abilities and generally impact cast time or costs.
Weapons and armor
As always these are all about the give and take. Every weapon will have a primary damage mode and likely a secondary mode out of 3 options. Physical attacks will have 3 modes, slash, pierce and bash. Armor will offer defense for each of the 9 possible damage types individually. Of course defense against one type comes at the expense of others, full immunity to one type of damage will leave you very vulnerable to any other type.
Trinkets and consumables
In this world most trinkets would have a small personal impact on the field effect, although some could impact social standing (I picture rep being very localized not a global thing). Consumables would have a larger effect on the field along with replenishing HP, energy and tweeking focus (the 3 player pools).
Solo combat is pretty familiar, the only wrinkle being the field effect. Most creatures use a single element, which you could use up and limit their abilities. More difficult creatures might generate more of an element (much like you can), however they do face similar limitations as the players (as they are built very similarly).
The group side is far more interesting. I mentioned my dislike of the traditional trinity, so now I'll offer the alternative. Every player has a base element (just to remind you of that point), it rears it's head here. That base element is also something you are nearly immune to. This makes any player, depending on the encounter a prime tanking candidate. Status effects (which boss mobs will dole out plenty) can only be healed by a counter element leading to healing being more by committee. Likewise with DPS. Essentially, the trinity exists within the encounter (although flexibly as some bosses would feature phase changes where their element switches) but party roles aren't fixed (at least not in an optimal sense).
Most of the progression is horizontal, at least from a combat perspective. Any power growth is specific and causes a deficiency somewhere else. Horizontally, you might be able to shift crazy amounts of attribute points around and build abilities from dozens of skill fragments but, you are stuck with what you brought to the fight. Something I hadn't mentioned until now is that those fragments can come from many places: the temples dedicated to an element, weapon trainers/experts, crafting, or various other organizations.
Vertical progression, is focused on social/economic matters. Whether it is membership in some order/society or a title of nobility, these would matter. I'm big on evolving worlds politically or otherwise. Maybe a rank member of the Order of Empirical Merchants can manipulate the comodity market, or a General can push invasion plans. I want those options and more to be availible to those that pursue them.
I know this is crazy long, and maybe just plain crazy, I'll leave that to you. Although if you'll allow the further imposition, any thoughts?