Even though there is a high chance this beta is what the launch build is going to look like, I noticed that is a good chance at least one more game mode will appear at launch.
The achievement MEDIC! is for a matchmade Elimination round, and there is clearly a revive mechanic involved. (as the description is just “Revive 3 allies in a matchmade Elimination round.”)
So with that, there is a chance for more stuff at launch, and possibly a game mode that is brand new for Halo MP.