I would love to see a good Naoto that is accurate to P4AU. (Complete with her Fate Counter.) (Update: looks like someone released a incomplete/beta version of her. Good.)
Other characters:
Nine, Kokonoe, Naoto, Terumi, Kagura and Azreal from Blazblue.
Al, Ignis, Mora, Anna, Saya, Ethica and Ein from Nitro+ Blasterz.
Ramlethal from Guilty Gear.
Tamaki, Touka, Multi, Arawn and Chizuru from Aquapazza.
Some Newgrounds characters.
And a Frollo that isn't a spriteswap.