I just started making a two player board game of X-Com. It's human vs aliens of course and I think the method I am using would work well for UFO: Alien Invasion multiplayer.
Two-Player Campaign
-Human function same as normal
Aliens-
The alien player functions as follows:
The goal is to totally destroy the X-Com force/or achieve a user defined terror point total.
Initally only has a few ships and can only send 1-3 at a time. Starting with the crappy scout/abduction saucer. The player can then send these on several different kinds of missons (terror, subvert government, scout for X-Com bases, attack base).
Terror points are gained for every completed terror mission and partial terror points are given depending on how much damage/civilian casualties are inflicted in the event that XCOM intervenes and succeeds or arrives late.
Terror points are used to acquire new ships and a base on earth. All ships are preloaded with random aliens and equipment by type. Bases on Earth generate constant terror points.
Aliens do not research, manufacture, etc, they can only buy more ships and bases. This is done to simplify the actual implementation of the alien side. Anything that they should be able to do should only be done if it is easy and for the purpose of giving the alien player more to do while the human player is doing stuff (maybe a terror minigame?)
The game functions as normal besides this with the alien player controlling aliens in all situations where aliens are present.
Time acceleration is controlled by both players or only the human player (it's a two player thing so if you're playing with someone who is a jerk and is screwing around with it all the time then they aren't worth playing with).
**This is probably the most fun we can get out of a two player campaign with the least amount of work**
