I have two ideas for gametypes that are based on monsters, like Invasion:
1. Nali Slaughter. Has anyone converted that from UC2 yet? If you don't know what that is, look here: http://liandri.beyondunreal.com/Nali_Slaughter Seems to be pretty easy to implement.

2. This one is a little harder and more complex. Basically it's an encounter and attitude gametype. You start with all the weapons, but there are no ammo pickups. Enemies drop ammo and powerups for you depending on their health. All of the players form a party. Monsters also form parties: Skaarj (Pupae, Queen, Fire Skaarj etc.), Mercenaries, Nali, Wildlife (Titans, Razorflies etc.) and maybe something else. Initially all monsters totally ignore you. You have a choice of what to attack. If you kill a Skaarj, the Skaarj party will be annoyed and will change their attitude, thus starting to attack you. If you kill even more of them, they will change their attitude to Hate and will hunt you all over the map. But then other parties that don't like Skaarj, for example Nali, will be pleased and will turn friendly, helping you to kill Skaarj. It also works the other way round: if you kill Nali, Skaarj will be pleased, but Nali Warriors will try to kill you. So it's kinda like a strategy-based invasion. Not sure what happens if you die, though...