A World Tour Mode along with a Final Fight side-scroller would be nice since we don't see them very often. The side-scroller can be implemented like Tekken Force.
Basically, include more single player modes and reduce the loading times.
When it comes to characters, we should get a Final Fight Season which can include: Sodom, Rolento, Maki, Haggar, and Carlos.
For other characters, Q, Eagle, and some of the EX characters would be refreshing but with Fighting Layer EX coming out, I don't see it happening anytime soon.