I'd like to see some sort of 'Fantasy Toontown' playset in which you can use your Sorcerer's Apprentice Mickey. Maybe the playset would come with a Three Musketeers Goofy and Kingdom Hearts-inspired Donald.
Also:
Jungle Book Playset with Bagheera and Mowgli, Baloo sold seperately. Although Bagheera would present a challenge in the toy box, as he's a quadruped and might not be able to use toy box tools and weapons and stuff. But if they figured out some sort of mechanic for that (maybe limit them like they do with Cars), there's no reason they couldn't give us some Lion King characters!
Darkwing Duck playset with Darkwing and Launchpad, Gizmoduck and/or Gozlyn sold seperately
Roger Rabbit playset, or at least Roger as a toy box character. And make Benny the Cab a toy box vehicle. P-p-p-p-p-lease!
Haunted Mansion playset that includes the Bride and one of the Hitchhiking Ghosts. Make a Mme Leota to sell separately. Yes, she'd just be a head in a crystal ball, but they could make it cool. It could be as if she had an invisible body. The ball would hover at roughly head-level and, even though there's no actual body present, she would still jump and drive cars and be effected by gravity as if there were a body. Tools and stuff would float in front of her. It would be pretty sweet.
Somebody mentioned Figment. Yes, he'd be a FANTASTIC toy box character!
It would be super unlikely, but I'd flip if they made a Doug playset. Put Doug and Patti in it, sell us Skeeter and Roger separately.