I think that your suggestion is pretty good actually. Only that there is not planet for everyone, only like 20-30 for each sides(depends on the amount of the players). The player can choose his first base location from any of the planets that are controlled by his race(x-com/aliens) But if he wants another base, then he must send a colony ship or something there (you could build a base to enemy planets, but...well, that is veeeery risky) So, what do you think?