Well, you could simply grab a spritesheet from an EB character you want to code and make a z2i version, this will also show your coding skills.
If the character is not even in EB, then it would be really difficult that an artist would like to spend time spriting a full sheet if there is no warranty the coder has the skills to complete a character.

While you would like to work on characters you would like to see. You have to be a team player. There are a lot of moments when coders are busy and another has to assist to keep things progressing.

"Play as Shadow the Hedgehog in a brand-new story campaign featuring never-before-seen powers and abilities that prove why he’s known as the Ultimate Life Form!
 
SONIC X SHADOW GENERATIONS also includes a complete remaster of the classic hit SONIC GENERATIONS, featuring newly remastered versions of iconic 2D and 3D stages with upgraded visuals and new bonus content."

It's possible this game won't have as many characters as BT3 (which has the 2nd most characters in any fighting game roster), but it's silly to think it wouldn't have a bloated roster. If the roster isn't obscenely oversaturated or unbalanced, it's not an arena anime fighter.

By the way are you planning to hack a rom or remake it for Ikemen?

They should have add Spiderman early as a secret character.

Spiderman was usual Xmen guest character in comics before Capcom had hold Xmen IP

Spidey has cross Xmen video games countless times from the snes, genesis, gb and the PSX activision XMen FG

As for Xmen characters

They should had gone with Sabertooth, Rogue and Gambit as early but it looks like Marvel Exec are the one that dictates the character in COTA since its more of a comic promotion for characters active in a saga that time but in Japan XMen 90s cartoons are the familiar and accessible to the masses than the 90s comicbooks becauseof import requirements

As for mechanics

MVC1 ExtraPartner would do wonders to XMen Cota like having other character like Cable, Jean, ProfX, Bishop, Jubilee, Forge, Mystique, Toad and etc that appeared in the cartoons.

For a Super XMCOTA'CE Turbo Penultimate Hyper Edition, I guess they would have added Gambit, Rogue, Sabretooth, and maybe Omega Red.  Longer arcade mode with chance of mirror matches.

I dunno.  I'm not super familiar with what is actually different from COTA and later games in terms of gameplay besides the obvious.  It's already a lot like the MVC games.  I guess COTA didn't have any of the special blocking from MSH or MVC, so maybe push blocking would make the cut in an update.

Tag mode as Capcom did later in X Men vs SF later.

Honestly... a better roster. Like, I get that there is an absence of villains probably because that game was based on earlier versions of the show, but even then there are some characters like Jean and Jubilee who could actually be there and have cool movesets. Hell, revisions with later seasons of the show including more villains like Mr. Sinister and the Hellfire Club would be nice, choices like Silver Samurai and Omega Red are so questionable considering how little they showed up in the show.

• push blocking (from MSH/MvC onwards)
• tag mode (from X-Men vs SF and onwards)
• special assist characters (from MvC1)
• EX versions of characters