There is already a game that does this. It is called Soul Caliber, it allows you add things to character models from a selection of pre made choices and voice modifications via higher or lower tones and so on. It, however cannot allow you to make your own moves, you choose a preexisting characters model coding and apply it to your alternate model. This, however has issue. Graphic flaws are common in some selections and animations even in professional level games and this whole idea would be very, very hard to do in a 2D/pixel art game that is made by fans, hell...frankly I would say it's impossible 2D wise without having totally un-stylizied and boring art work/ moves.
Because this will be a 2D game, the idea won't work too well. Playing on perspective and stylizied animations along with the fact that drawings can't be alternated and played about with like a 3D model means that you would have to basically just re-skin existing characters and have a rename them option. The quirks of each characters animation will still exists along with their drawings, not unless you also get artist/ drawers who are willing to make entirely new doubles of the original characters that lack extra details or animations that interact with those details-hats...scarfs. Blades. Chains on blades. Side kicks, animals, intro characters with them, powers specifically to that character, personality traits and expression obviously belonging to that character- it works with soul caliber because alot of their animations are anatomically correct- as it is with 3D- and thus has no styling or visual play on their existence, along with that a lot of their moves besides their specials are ambiguous and can really be applied to anyone besides the most latest additions thanks to that ambiguity.
Hell, the designers themselves know that those animations are ambiguous because they reused character styles/ animations for bonus characters with only minor changes in their earlier release.
I'm not saying it's can't be done, but asking a serious company with multiple paid professional for this kind of a big deal. It's also really unnecessary. If you want to make your own character into a basic fighting game then Mugen offers you pretty friendly tools to do that. Custome moves feature is also begging to be exploited and cheap, as well as hard to maintain and code/ look artistically correct and pleasing to multiple applicable subjects. Either the costume generated lot would have to be blatant re-skins of the originals or blank, boring to look at un-stylized generic ponies.
Yes. I'm aware Street fighter reskin their animations but they are still difference that have to be entirely redrawn and animated Plus, street fighter is outdated. There...I said it. Street fighter releases have been laughed at by modern generation fighting game community for a while now. Skullgirls kicks the crap out street fighter and their not nearly as big and well funded as street fighter, hell. I'd say it has better visuals too which is arguably the only thing decent about that game-well..ok..it does have some jamming music at points- The only cool thing this generation street figher have to offer is that focus attack mechanic.
@Lunar Red, I'd say focus on your combo mechanics and balancing your characters first. Worry about cosmetics and customization last. Focused characters on their mechanics is better. It's best you put your effort into the mechanics into a few rather than give all that effort to making essentially a hard to maintain/ most likely balancing breaking device that probably won't even that well made due to the level of difficulty it would need and lack of good reference or examples to look up on.
Also..what the hell. I'll apply a character later too. Do we absolutely have to be ponies or can we be original characters if their stories are decent and sensible?