The best thing possible, following the Resident Evil 5 approach (Which is the best selling game of capcom through all times) one should add coop through everything.
Quick suggestion would be to have Ryu and Momiji through everything, and make it a possibility for coop over the net or select one of them with an AI. Look to RE5 for things to implement. Once you have completed it you can replace any of them with Kasumi or Ayane, just like in the regular version.
Also, maybe one of them (or everyone) could have a slightly different combat system. Look at Batman, MGS Rising or Assassins creed 4, the "counter button" + directional + follow up command would make the combat feel more epic and fluid. Right now you feel like crap in NG until you get a finish move and get the cracking going. (Powerup (hold square) and release is something I feel I must do waaaay to often, and it is one of very few moves I feel is worth doing as it gives that heroic crush to enemies reward feel when you hit)
Look to the battle of Jet Stream Sam for how to do it. It is actually strange that DoA and NG is related when countering has such low importance and usefulness.