So after taking a look at the game modes listed, I think it would be a great idea for Rebellion to add in a few more game modes through use of DLC. Other than MP maps of SP expansion which is a given, I hope some fresh, new game modes can make it into the mix! Here are some ideas!
Predators
Basically, it is an all Predator FFA match. For each player, they are given another Predator to hunt. Let's just call them his "target". The only way to score is to hunt and kill his target. Every player is randomly given a target. So it would be possible for everyone to have a different target, or everyone to have the same target, or some to have the same with others not being the target at all, etc... You hunt and kill til the end. You only score for killing your target, and gain nothing or possibly even lose points for randomly killing players that is not your target.
AvP Hunt
Basically think Predator Hunt, except you add in a single Alien to the mix. Same rules as Predator Hunt, except now you have an Alien to worry about as well.
Get To The Choppa!
I couldn't resist naming this game mode that because the name of it is exactly what you are supposed to do. A team of marines must get from point A to point B with a couple Predators stalking them. One life per round. All marine players start with only a pistol and maybe have a single pulse rifle for one of them to grab. Those who make it to point B gain points, while the Preds score points per kill. Maybe have 3 -5 rounds total with each player being randomly selected as Preds. Would be really frantic if you ask me!
Command
2 teams. Aliens and Marines. Each team has a leader. You are allowed as many lives as possible as long as your leader is alive (he/she only has 1 life). The whole point is to kill the opposing team's leader so the enemy can no longer respawn, and finish them off for good. Maybe have 4 rounds, with each team being the Aliens and the Marines twice. Think Gears of War 2's "Guardian".
These are all just simple ideas I threw together in an effort to breathe new life into the game, unless Rebellion already has ideas up their sleeves. I think these would all be great additions to the multiplayer aspect of the game, not focusing on new maps which is a given. Feel free to throw out any other unique ideas. Try to make them stand out. And I kept Evac out because I want to hear new ideas. Hopefully Rebellion takes a look at this because I think they could use some more unique game modes.