When I read this, all I thought of was a variable "date" on these big events... but I like Pete's idea, something a bit adaptive. It could be simple, like tracking a kill ratio and basing alien numbers, equipment, and skills off that. Or it could be complex, where a player packing all sniper rifles all the time will find himself faced with grenadiers heheheheh. That could lead to intellectually ambushing the aliens: giving them a number of easy victories, then ambushing their lax and poorly armed forces (maybe they're spreading their heavy arms out now that you're suddenly easier to defeat--because, maybe, they have other enemies in the galaxy to worry about). This could be like a classic Mongol mangudai, a false retreat which is actually an ambush. Hmmm...