My 2cents on 2d fighters.
-Fluid mechanics, it needs to have some type of pace or rhythm to it.
-Diversity of play, characters with difference styles and things that make them unique compared to the next character. People end up identifying with certain characters for whatever reason when you have enough of them that are different. Some people want to play big strong dudes and some people want to play small school girls.
-Having multiple fighting game mechanic options, like in soul calibur, you can parry, JG, block, sidestep, auto gi, backdash, jump, and interupt. Allowing characters to get around certain things that would be mindless lock-down otherwise. Having multiple options also makes the game more rewarding because a right read on an opponent ends up giving a lot of reward.
-The graphics don't have to be amazing, but they need to make sense and not look sloppy. Like a minimalist game can look amazing, there are graphics of games from SNES that still look amazing today.
-Music generally is just background music, done well it can add a whole lot to a game though.
-The fighting game system just needs to make sense, like you are incrementally working on defeating the opponent. People want to know why they won. Random deaths and weird mechanics that instantly kill and opponent are generally unwanted. Exceptions are games like Marvel where there are multiple characters.
-Keep the storyline simple and straight to the point. One's that are over ambitious never seem to work when it comes to fighting games lol.
P.S. Let us know when you finish, I would be interested in playing it.