This sounds like fun! It was a topic I've been meaning to do as well =P
I'll list some of my own:
Daimon:
Have him recover faster from his ground-shaker feint so that he can do a normal, followed by a feint, then another normal. It is literally a feint cancel. (Of course, he would only be able to link in two normals like this so as to not make him too cheap)
Terry:
Invincibility frames on the startup of Rising Tackle.
Joe:
Change the animation of his EX Slash Kick to that of his RB2 strong Slash Kick. (More of an aesthetic change than anything)
Kensou:
Change his standalone qcf+K to be a regular flying kick. (Like a Shingo Kick)
Buff the damage and chip on his hardly-used dp+P.
Mai:
Give her an EX version of her d~u+P, which allows her to cancel it at anytime with a normal, makes her follow-up multihitting, and allows her to cancel out of it with any aerial move like air qcb+P, her (EX)DM, or her NM.
Raiden:
Allow his EX Head Crusher to be super-cancellable to his command grab DM.
Makes his Crazy Train an "impact" DM. (That is, when the first hit makes contact, the rest of the DM will pull off; similar to one of Hinako's DMs in '01 and '03)
That's all I can think of for now.