Some experimentation I'm making... 4 level bases, basically. Here's some concept map: adding severs below and air ducts/service corridors above (some facilities would stretch for more than 2 levels, though). It would mean more FUN when defending bases... Naturally, going for it would mean an insane amount of work since all RMPs would have to be repaired (moved 1 level up, which is AFAIK possible only by remaking them from scratch...).