Possible return of Ruukuto?
I know I shouldn't get my hopes up, but...
Also, Rin Satsuki for final boss. Because she needs to actually be put into a game at some point.
Anyway, I'll be serious for a moment... I'm very happy that Meiling is finally back. Sanae could potentially be a very interesting fighter, so I'm also glad about her appearance.
As for Cirno... Finally, a low level boss as a character in the fighting games! Not the one I'd hoped for (three guesses as to who I wanted, and your first two don't count), but I'm still pretty pleased by this development.