I think Vader could be next on the list as they could injection mold it and stick a voice changer in it.. fits their MO so far. That or an episode VIII tie in as Fener says.
Fett seems very unlikely to me.. you need to look at their past ones. They're easy to make voice changer toys. Darth Vader has the size and space for a voice changer, and a relatively simple to mold and inject design where as Fett's doesn't. Vader would come out Black and glossy while Fett would need painting and assembly. If I was Habro, Fett just wouldn't make sense.
For people who want Fett's, Targets $30 Fett's got it right, I don't see Hasbro getting it any better.
Other possibilities I can imagine or would like would be the new FO Tie fighter helmets, Deathtrooper and/or Scariftrooper, Rebel Helmet, etc.
