So here's what I'm thinking for some characters
Vergil:
-No Beowulf or Force Edge. Trish has Sparda and Dante took Beowulf and stashed it somewhere. He should still have more swords though. It'd be kinda cool if he replaced those with Ifrit and Alastor. Unlikely, but cool.
-DmC had the right idea in giving him Doppelgänger. Also give him Quicksilver with on the fly style switch. Dark Slayer, Quicksilver, and Doppelgänger. He'd be a real mobile and zoning character. Just all over the map.
-A couple more "guns" would be good. His Summoned Swords is great and all, but I'd like to see some extra stuff. More magic, obviously. Dunno what, though. Trish's lightning tricks and traps from MvC3?
Trish:
-Really needs some heavy weapon variety. Some classic stuff. I think it would be pretty awesome to just give her classic weapons. Use her as a rehash character in some ways. Take all the lightning-based weapons for instance. Giving her Nevan and Alastor would be super cool and I'd be down to clown with that.
-Giving her transformation weapons in general seems like a solid idea. Sparda and Pandora would be the biggest, considering you have a transforming sword and a transforming gun. Of course, this would be a full Pandora instead of the neutered tech that we got.
-Translating her from MvC3 wouldn't be a bad idea. I'd like to see her lightning magic make it in, and maybe they could even do something with her flight tech.
Lady:
-Honestly, there's already a really solid base here. Give her Spiral, since it's not a demonic gun. Try to keep her away from demonic weapons and do much what Vergil did, although she needs a far more versatile sword weapon. The Kalina Ann (Bayonet) doesn't even touch Summon Swords. Not even remotely. Having her as a reverse Vergil is a really amazing base to build off of though. For sure.
Nero:
-Increase the Devil Bringer versatility, instead of having pre-programmed throws, allow him more control over the types of throws he does using inputs. Also allow more direct usage of the Devil Bringer e.g. throwing some punches. I'm not sure about giving him different DB "styles".
-Maybe giving him different guns equipped to the D-Pad and then swords equipped to the right trigger while still leaving the Exceed function attached to the left trigger, giving all the swords a secondary function.
-Having more unique and complex DTs with their own move sets is also a great idea as well, rather than them being just a power-up (albeit, a cool looking power-up that sometimes gives movesets a little extra oomph). Like the DTs from DMC2, maybe?
Just some base ideas I had that I didn't want to bog down and drag the leak thread off-topic with.