I have commented on this previously, but I have a new suggestion. That aliens invade in waves and their bases remain spread out, so multiple bases would group and attack you but they don't creep right up to your base (or become a continuous hive), and when they build a base it does not re-spawn ...

BTW: GREAT GAME, around 200 hours no time flat... 4th full game, experimented with multiple settings. no mods yet.

Aliens Creep from their original bases, then spread like wildfire around the map.

This does several things.
1.) it forces the players to take action before they are enclosed prior ...