I don't know if Kofi is main event, belt holding material. But, I have anxiously been waiting on a heel turn from him for quite some time now. If a turn were to happen, I would be interested in whatever he was doing.
I'm obviously biased because I'm a huge fan, but I would think Ziggler is a top candidate out of this list. When he was on his own for that 5 or 6 week span before AJ and Big E climbed aboard, he was really hitting his stride. The locker room brawl with Cena, some really solid promos, and as always, awesome matches. Like Vickie did eventually (not at the begining), I think AJ too is holding him back a bit. His character is always tied to a woman, it'd be nice to see him completely on his own for a solid chunk of months.
Antonio Cesaro, Wade Barrett and in time Kassius Ohno and Bray Wyatt.