This is just a suggestion, They may sound Stupid so be as brutal As hell
Ranks - After killing a certain amount of aliens, or if your an alien in a game, You gain a rank, First one is Private for Human, Prawn for Alien (Prawn Means Helpless Bottom Feeder) Next is Sargent, Predator for alien. Lutenant, Pouncer for Alien. Captain, and Tyrant For Alien. Last - General, Berzerker For Alien.
Game Mode - This is inspired by the game Lost Planet 2. You have one big Open Area, And You choose Which Big Alien To Kill, Or Huge Machine for Human.
You have a Giant Alien that Cycles through the map, And Kills All the humans, But The humans Have to shoot a certain part of it to bring it down. For Aliens, They have to take down a certain part of the mech to kill it.
Retarded Yes. Smart, Maybe.

Wut ya think?