Doctor Light
Abilities: Flight, laser, manipulation of Green Lantern objects.

If you don't know, Doctor Light is a villian with the ability to manipulate light. As such I think he would be good for no other reason then to have someone else able to build the green sparkly LEGO objects. One of the things that really let's Sinestro down, espiceally as he's Green Lantern's Nemesis.

The other bonus of Doctor Light is that he has battled all the heroes at one time or another (in comics/tv)

Gizmo
Abilities: flight, magnetism, laser

I think Gizmo would be good because their isn't a villian for Cyborg in the game even thou he's part of the story line.

Ma'alefa'ak
Abilities: flight, laser, invunerability, sensor suit.

Ma'alefa'ak is Martian Manhunter's evil brother. I'd add him for the same reason as Gizmo for Cyborg, Martian Manhunter needs a nemesis for himself.