Today Midway has released the achievements list for XBox 360 for his game: Mortal Kombat vs DC Universe. From this achievements we can find the name of the last four warriors that will take part on this war. They are:
MK Side - Baraka, Shao Kahn
DC Side - Lex Luthor, Darkseid
And they talk also about the boss, his name is Dark Kahn and is a fusion of the two bossess of both side (Shao Kahn and Darkseid). From this list we can also see that the game will have a Story Mode with 6 chapters for both side, an Arcade Mode with ladder that go from novice difficulty to max difficulty, and a really good Practice Mode with all combo to learn (Expert Combo).
This is also revealed on the last MKast #6 up on WorldsCollide.com:

* They talk about Joker and Kitana's Special Moves, and about the edenian princess she will have, other than her old moves, also the rolling of Mileena and a Teleport very useful in combo;
* There will not be similiar moves, so if you remember old videos, Batman and Scorpion own the same Leg Grab, so one of them will lose this move;
* They explain in detail the Rage mode and the function of Combo Breakers;
* Voice acting is performed by real actors;
* In this game there will be a leaderboard online, with ranked match and level upping;
* MK and DC warriors will have both bios that explain their situation on this game;
* They revealed some new arenas: Gotham Streets, Gotham Rooftoops, Space Station, Special Forces and Senate.