MOTUC Buzz-Off, Stingrad and Queen Andreeno
Andreenos has quite a number of named characters, and I hope we'll see them all in MOTUC.
Buzz-Off is probably a given in the line, provided the line survives for another year or two. I'm sure the 4HM will make him look really great.
Stingrad would just be a repaint of Buzz-Off with a different head and offered as a bonus character. That way those who don't want him could pass on him.
Queen Andreeno is a character I'm looking forward to. Not only does she adds another female to the line, but with her, it'll mean we're another character (after King Randor) down to completing the Eternian council.
Edit: Bumblor is a bit on the wide side and would probably require new parts, so I don't see it anytime soon.