Hmmm, to me 200-300 active abilities just seems like too much in number.
Even well-established systems like D&D don't offer so many in computer games most of the time from what I have seen. From my perspective it is much better to work through with fewer abilities that feel unique and impactful than having 300 of which 285 will either never be used, or only used once to test them out.
If you're talking about a card game then I understand you would want more than 15 abilities, that's sure, but I think you may want to start smaller than 200 anyway.
My thoughts on this is that the higher number does not always generate a better gameplay still. I would suggest this off the top of my head. Take a real deck of cards (52 cards) and try assigning abilities to each of those. That's already quite difficult in my opinion, but would show you whether you need 50 or over 200 quite quickly as you would then also determine the time it takes per 'deck'.
Finally, balancing over 200+ cards would be a very difficult thing to do.
For reference (regarding Slay the Spire:
https://en.wikipedia.org/wiki/Slay_the_Spire
Don't think of Feats as Feats in D&D
My Feats are really just "active abilities" of all kinds. This includes spells (though, in my system, they're telepathic powers).
D&D has hundreds of spells, for instance.
This will become clear once I finalize my system and the first iteration of my character generator. I'll send a copy to anyone who's interested - but it will be a while before it's ready.
For my system to make sense and be properly diverse, it really needs a large number of active abilities. 200 would be the absolute minimum.
It's one of the most important aspects of satisfying progression and interesting combat and non-combat gameplay - in my opinion.
Having a reasonable pool of active abilities for each skill subcategory is integral to my Crux system. For instance, if you focus on Ranged Combat and specialise in "Handguns" - and you go for "multi-targeting" (which is going to be part of the handgun specializations) - then you need more than 1 or 2 active abilities to support that. Essentially, each weapon specialization (I call these Foci in my system) needs a small handful of active abilities to support the intended build and gameplay. Each of them will have around 5 abilities, which isn't THAT much when you think about it.
It's no small task, to be sure - but the difficulty isn't really going to be the actual amount - it's going to be coding the necessary systems to support the kind of effects I need in feats. As in, I don't want active abilities to resemble each other much at all - so they need to feel as distinct as possible.
Thankfully, by going the "card visuals route" - a HUGE part of the workload connected with traditional active abilities is removed. Because I don't need custom art or animations - beyond a few basic things, that have yet to be determined.
For instance, weapon feats (like, say, Sniper rifle feats) will all have the same "target line animation" when you trigger the ability and pick your target(s).
So, it's not as much work as it might seem.
I have a large variety of mechanics in place to be affected by feats - and the challenge is really to get those mechanics working in tandem with the combat system.
From that point on, it won't really matter if I have 50 feats or a 1000 feats - because all feats will deal with the same mechanics.
Well, it takes a LOT of work to come up with a few hundred interesting and distinct powers - but it's the sort of thing I actually CAN do.
Also, I should probably add that I'm not designing a system that's supposed to be "balanced" in the modern sense of the word.
This is a singleplayer/cooperative game - and while I will do my best to have all traits, specializations, feats, skills and stats be reasonably attractive, I have no illusions that people won't find a ton of exploits.
That's part of the fun of games, in my opinion. I'm essentially a huge fan of diversity and creative combinations of powers and skills.
So, a lack of balance is intentional - even if I don't want it to be downright broken.
But such a thing can't really be avoided once you enter the realm of sufficient complexity.
My "solution" to balance problems is to have a system that's rich and complex enough so that there's always going to be more ways to min-max and there's never going to be "one thing" that's better than everything else.