Right now there's 3 modes

1. Normal (Campaign)
2. Deathmatch
3. CTF

However, how about Invasion

Choose a Deathmatch map that's in 1 piece (as in, no separated rooms connected by teleporters), set up the marines wherever they wish. One player is the Invader player and he creates his deck of nothing but spawn cards (events aren't used). The objective is simple. The marines must survive for as long as possible. The invader player gets infinite spawns (if the deck runs out, reshuffle and re-use), and the marines don't respawn. Track the number of turns the invader player used using the frag tokens. The more tokens he gets, the more winning the marines. Test it and comment on it.

I like your idea a lot. A couple ideas to add some spice using your same concept. Rather than just have the marines survive as long as possible, why not add some objectives, such as a room the marines must assault or defend. Either side can earn victory points by occupying that room, and when a certain number of victory points are reached, one side wins. Or, how about the marines winning if at least one of them survives through an entire spawn deck...once the spawn deck runs dry, game over. See if the marines can last that long, but if one of them dies, that marine is done. Sort of a last man standing concept. Just some ideas. I'll try it out.

How about the marines need to survive 2 rounds of the spawn deck? Once the deck is out, spawn a CYBERDEMON when the last monster is dead. When the Cyberdemon is dead, reshuffle the spawn deck. Once it runs out again, spawn another Cyberdemon. Once the marines kill 2 cyberdemons, they win.

Sounds good...I'll give it a try.

I just had to place the cyberdemons in somewhere, to be the Halfway-point-Boss Battle and the End-Boss Battle

PS: The marines should not get any health. Either remove the health packs, or don't re-spawn them if taken.

Maybe I am a bit slow, but i don't see any chance in hell of marines winning that without health tokens. Unless you give them some seriously extra health, and really big guns. and even then...Two rounds of spawn cards, and two cyberdemons is a lot. so... Am I missing something? A rule for these types of play maybe?

Maybe give them healthpacks, but don't respawn the healthpacks when used. The marines would just camp on the packs if it respawns.