I would definitely welcome a Superman series. Like I said before awhile back, long overdue. There should have been a tie in with Man of Steel and Superman: Unbound but since the sequel to the former is coming up, they should really consider getting Supes another animated solo series. But I imagine they'll just make a Superman/Batman series. Not that it has to be run by Bruce Timm but I admit it would be interested to see how he would approach a Superman series now that he's done a couple of movies namely All-Star Superman that changed a couple of his thoughts on the character.
The Flash is another one I think is long overdue. He's got a cool rogues (and sometimes loveable) gallery like Batman. The show would write itself.
I know it won't happen but I still wish James Tucker would have been able to make this Wonder Woman animated series.