I remember that in Doom 1 and 2, monsters will sometimes attack and kill one another. In fact, in some scenarios, it's a very viable strategy to whittle down the hordes by sowing discord. (aside: does this happen with Doom3 AI?)
This experience set me thinking: after all, the invaders are nasty creatures from Hell and shouldn't work together as 'harmoniously' as the marines. I think the best way to simulate this would be to have 2 invader players playing competitively against one another.
Maybe the following setup can work (for scenarios where marines need to get from point A to point B):
4 players: 2 Marines vs 2 Invaders (2v2)
Marines choose their colours and invaders will play only the corresponding colours
Marine and Invader turns will alternate: eg. Red Marine goes first, then Red Invader, followed by Blue Marine and finally Blue Invader.
Invader players start with their usual hands of 5 but will each draw only 1 card from the common Invader deck in their turns. Each invader player may play only 1 spawn card in his turn. Event cards are played as per normal.
Invader players may not reveal their hands to one another, and they may not trade cards.
For new areas, the invaders will place lurking monsters (lurkers) of their own colours first.
For a white lurker, any invader player may place his monster IF he discards any one spawn card. He may not play any spawn cards in his next turn. The card discarded in this manner does not need to match the monster type specified by the scenario.
If no invader player claims a white lurker (by discarding a spawn card), then it's not put in play.
In the event that BOTH invader players are claiming the same white lurker by discarding spawn cards, a bidding war will start. Invader players will each secretly decide how many event and/or spawn cards to discard and reveal their bids at the same time. There is no minimum bid.
The player who has bid more cards will 'claim' the white lurker and discard the cards put on the bid. If there is a tie, the outcome will be decided randomly (coin flip?), and the successful 'claimant' will discard the cards, if any, on the bid. The player who loses the bid keeps his cards.
Ok, an example:
Area 1, Scenario 1 has following lurkers:
1 red zombie (claimed by red invader)
1 blue zombie (claimed by blue invader)
1 green zombie (not in game)
1 white zombie (red invader player discards a spawn card to claim this lurker, blue does not object)
1 white imp (both red and blue discard spawn cards to claim this lurker, so bidding starts: red reveals that he's willing to discard his Darkness card, while blue reveals a Zombie spawn card - it's a tie, so coin flip - red wins the flip, discards his Darkness card and places a red imp on the board. Blue keeps his Zombie spawn card and has to be content with his single blue zombie in area 2)
The bidding mechanic might hopefully offset the power of potentially 2 spawns per round and an effective starting hand of 10 cards (5 per invader)
When the invader deck is depleted, the single frag point will be awarded randomly to one of the Invader players.
Modified winning condition for Invader players:
The first invader player to score 5 frags against the marines will win the game.
Note that in 2v2, the invader players are playing against one another, as well as against the marines. Monsters can attack and kill monsters of another colour. Thus the invader player needs to worry about the marines, as well as his 'ally'.
I hope the above suggestions will create interesting 3-way situations. Not sure about balance though - will playtest and report when i can.
Re:2 Marines vs 2 Invaders?
The white monsters need to be assigned to an invader player wihtout having them discard a spawn card. I think around half the monsters in the game are white. Youd never get enough monsters without them.
Re:2 Marines vs 2 Invaders?
Yup, that's one problem i was worried about when dealing with the white monsters ...
Basically, with the above mechanic, I'm taking a Spawn card (or more, in the case of a bidding war) out of 'circulation' for every white monster - which will lead to a shortage of spawns (and cards) over the course of the game.
Another issue is probably the fact that the colours are not represented equally in all areas.
eg. in area 6, scenario 1:
2 blue imps
1 red zombie
1 green zombie
2 white demons
2 imps = 1 zombie? arguable.
Definitely needs playtesting to see if this makes the game too easy for the marines