Too braindead to respond to the rest, but I've decided this is an amazing idea. You know, compacting everything into one ability like that. However, I've got a few different concepts in mind for each.
This reminds me of another idea: Learnable item abilities. They were in FF... uh, the one with the monkey tail guy.
Anyway, the point is you get an item, let's say a sword. And on this sword is the Critical Strike Passive, or the Wind Slash Active, or whatever. While this item is equipped, you gain access to said ability.
But while said item is equipped/whenever you use said ability, you gain experience in it. Eventually this means you master the ability and can use it without needing the item any more.
The obvious problems are that it can be kind of gamey (I know
I'd equip items based solely on whether I could learn an ability from them and not whether they or their taught abilities were in any way useful) and that it's moving away from the simplicity I assume you were going for. Still, I like the notion of learning things permanently like that.
Active Gonna Murder Shit Concept One: Ability Slots. So! At the start of the game, the Active Combat ability has only one use. Maybe it's like, a Big Fireball that you can use instead of attacking at some cost (SP? Time?). Either way, if we narrow down all a character's combat abilities into one thing, it doesn't allow a lot of room for advancements. That's where Ability Slots come in! Think Path of Exile's linked abilities: on-base characters can craft certain items that can merge with your active ability to add an effect. One idea might be a Phase Gem that teleports the enemy away from you in a random direction. This would be awesome for a fireball, letting a ranged character continue their kiting, but not so much for a melee strike, giving weaker enemies a chance to run.
Active Gonna Murder Shit Concept Two: It's literally your sword! While I'm not sure compacting the game even more is a good idea, your chosen Active Ability could just be your normal attack, and any modifications made to it modify that, too. If combined with Ability Slots, too, I would likely let the player change out slotted items on the fly so they don't get stuck doing ONLY one thing ever.
This is kind of interesting as is, though, especially the swapping things out bit. I imagine you'd want to limit the number of Ability Slots, but that'd mean every player essentially had, say, three separate abilities, they were just all the same basic thing their class did (blast things with fire, hit them with a sword, etc).
It could also get somewhat interesting when you consider what happens to utility abilities under this system. First assumption would be that they'd die out; you can't have a healing or wall-jumping ability because you can't do anything that isn't hitting something with a sword. But what about using one of those rush forward sword attack things to jump pits? Or having a healing strike type spell, wherein you heal allies as part of attacking enemies?
That particular last point could also deal with some issues healing tends to have, like where your real HP are some number based on your healer's efficiency or where you heal up after every fight because SP regenerates for free. Which I guess could be a problem if that's how you
wanted the game to work, but maybe you could just let them sit down and heal innately at that point.
Passive Ability Idea: Actually I forgot my idea for this!
This one's actually kind of tricky, because you've already got attacking and its associated effects under Active Murder, and you've already got loot and to some extent exploration under Loot Finding. Unless you can swap out one or both, dealing with either would be kind of redundant, since it could have just been an innate effect on your actual ability.
So I guess what's left is defensive, situational, and possibly exploration or movement abilities. Even most of those are in some danger most of the time, due to the possibility of defensive or movement-oriented attack specials, but could still at least potentially come up if you weren't attacking that round for whatever reason.
Situationals are the real option, though, since at least in non-attack terms it usually makes more sense to break those off. Execution-style abilities, for instance, would still be in danger of begging the "why isn't this just on my sword?" question. Benefits at low health
could function the same way, but especially if it's a dodge bonus and not something special your attacks do, they might make more sense being character-innate.
Of course, there's also simplicity, and if you can swap out your base abilities and/or passives there's that. Saying Fire Blast blasts things with fire and Executioner grants +1 damage vs heavily wounded targets might be cleaner and would certainly be more modular than saying Fire Blast blasts things with fire and has a +1 damage bonus vs heavily wounded targets.
This is How I Find Loot Idea: Oh, oh God. Okay, here's my idea for this. Each character's loot-finding ability is linked to a specific type of puzzle component. Maybe it's pushing really heavy boulders or creating wind storms. Either way, every shard would have multiple puzzles that could only be solved using certain combinations of these abilities. Basically, when you choose this, you choose what type of video game puzzle you're good at solving
I'd say "Oh, oh God" sums up my reaction to this fairly well.
Also what happens if we get five block pushers in one party? :x
Oh god those block puzzles that need to be done quickly because they reset after a certain number of moves/time. DAMN IT, answered my own question.