What I personally like in a fighting game:
Strong, freeform movement. Smash, KOF, GG, the like.
Strong characters that can do crazy stuff.
Universal defensive mechanics to ensure the crazy stuff doesn't result in checkmate situations as often. As a lesson from SF4, it is important that these defenses not be stackable. VF can handle pretty crazy defensive OSes as far as I know, but for a 2d fighter I'd say try to make the defenses not be stackable. Focus absorb OS late tech into invincible backdash is nasty stuff.
A sharp feel to the controls.
That's it, for the most part. As far as systems to study goes, I'd recommend KOF13. That game is one of the most beautiful and solid pieces of system design I have ever seen.
I think a small amount of imbalance is actually very good for the game, because people enjoy different things. I derive enjoyment from having better game pieces than the opponent, others want the low tier uphill climb (not just as a "I play Dan so if I win I'm great if I lose lol didn't matter it's Dan" type of OS) or to play the uncommon stuff.
The flipside is of course that you don't actually need much imbalance at all to do that and intentionally trying for it can backfire dramatically making the stuff that was intended to be a tad too good or a bit weak either truly godlike or Ultra Deejay tier. But overall I think balance is quite overrated. I wanted to play the most balanced games initially but in the end that just meant playing AE2012 which just wasn't that fun. The more imbalanced games turned out to be way more so.
The optimal size for a character roster depends heavily on how the game's matchups are IMO. Like, there are games where matchups are "heavy" - there's a ton of character-specific esoterica to learn to play competently. SF4 or Guilty Gear being good examples. Tekken too, maybe.
I don't think a large roster is a good thing in those kinds of games. It just gets to a point where you can't actually know your matchups well because there's so insanely much stuff to learn about each of them.
KOF13 style design has simpler characters (less buttons, less close/far variations), the game system is a larger part of how the matchups play out (instead of the gameflow of each matchup being determined mostly by the characters like in SF4 and GG) and there's way less character specific stuff in general. Being able to "play KOF" is the main thing. That kind of design can better support a larger roster IMO.
I like both the general simple just play the game style and the matchup-heavy style a lot, but I really really would hate seeing a 40-character GG roster for example. That kind of stuff just makes me want to quit.
