I'd like to see the Question. I highly doubt he'll be playable but I hope he has a huge part in the story.

I think Gorilla Grodd could be a lot of fun.

Darkseid and Starfire would be at the top of my list. I also wouldn't mind seeing Superboy Prime.

I'd prefer the Renee Montaya version, but both could be included as variants of one another.

Anyway, some of the characters I'd like to see below.

Mr Mxyzptlk
Gorilla Grodd
Cheetah
Booster Gold
Lobo (as a part of the storyline this time around)
Ambush Bug
Bizarro
Circe
Poison Ivy
Clayface
Tommy Monaghan (from Garth Ennis' Hitman series)
Etrigan the demon
Mouse Man
Miss America (obscure Golden Age character)

Red Hood. Jason Todd needs to get a chance to shine on the main roster of something.

GRODD. Come on hes a telepathic gorilla.
Power Girl.
Cheetah. Need some more WW villains.
Atrocious. (But Bleez is acceptable too)
Starfire.
Darkseid.
Big Barta.
Mr Miracle.
Bizarro.

Power girl
Cheetah
Gorrilla Grodd
Circe
Firestorm
Brainiac
Livewire

The Mask (that the Jim Carrey movie was based on) as a guest. A few other worthwhile guests would be Witchblade, Spawn, the Predator, the Punisher, Venom, Carnage and the Superior Spider-Man.