Here is one proposal, which is not really "reinforcements" but ...
what if an alien ship crashes, and 2 aliens move away to scout the area?
then the player team attacks the ship, and the two aliens rush back to help their alien-friends out?
Not sure if that would be worth any effort but here a few ideas why this could be fun:
- Aliens could suddenly appear at the end of a map, an area the player thought he cleared it already
- Aliens that are part of such a small scout team could actually survive that onslaught, and decide that they
will terrorize a nearby village or something, so the player needs to get them down before they go rampant
- Aliens that can escape this way could perhaps engage in another mission lateron to help out other
aliens
Not sure if that is worth any effort though, but i thought it is fun to just write that down