So I’ve heard that true portals are on the way. Here’s an idea for a game modifier which makes use of them.
Aliens do not need an overmind, but they only ever get to build a finite number of eggs that do not regenerate health. Deconstructing an egg simply places it into an internal egg queue, so you can’t just rebuild an egg if it gets low on health. There are no changes to the human team.
When the game starts, an exact copy of the map is made. It is initially inaccessible by either team. Aliens are able to turn certain mapper-specified planes (cross-sections of vents, doorways or hallways) into double-sided portals to the copy of the map or remove existing portals they have created. However, once the first portal is placed, no part of either map must ever become inaccessible for more than some amount of time. There should of course be other restrictions on placement and removal of portals for the purposes of game balance, but I won’t go into specifics right now.