Heroes:
Zatanna
Raven
Black Canary

Villains:
Queen Bee
Poison Ivy
Star Sapphire

Stages:
Oa
Atlantis
Arkham Asylum

Heroes
1. Green Lantern
2. Cyborg
3. Raven

Villains
1. Atrocitus
2. Lobo
3. Black Manta

Arenas
1. Oa
2. Atlantis
3. Arkham Asylum

Zatanna
Batwoman
Green Arrow

Poison Ivy
Circe
Star Sapphire

villians:

Deathstroke
Killer Croc
V

hero:

Raven
Beast Boy
Azrael

Stages:
Arkham Asylum
Gotham City

HEROES:
Firestorm
Aquaman
Raiden as a Mk guest, as possibly hinted at

VILLIANS:
Clayface
Mirror Master
Starkadr

STAGES:
green Lantern Planet, Earth-One
Phantom Zone
The Batwing

1. Red Robin
2. Red Tornado
3. Super Boy

1. Mongul
2. Black Atom
3. Black hand

Heroes:
Zatanna
Black Canary
Aquaman

Villains:
Deathstroke
The Joker
Lex Luthor

Stages:
Arkham Asylum
Atlantis
Gotham City

Besides what's confirmed...
Heroes:
Martian Manhunter
Hawkgirl
Starfire

Villains:
Deathstroke
Bane
Doomsday

Stages:
Gotham City
Oa
Arkham Asylum