What about... CM round having 2 stages?
First stage is proper survivor vs benos. Most of the server spawns as benos and survivors. The catch is that survivors are designed to lose, their only objective is to survive (hiding/escaping/fighting). Some people spawn as Almayer crew (CIC staff, maybe MPs, researchers, OT, mass seargeant, RO, CT and MT, hell, maybe even few PFCs if they want) and have a lot of time to RP, prepare plans, prepare food, research and create explosives.
Second stage is classic marines vs benos. Survs that died this round can respawn as marines and benos.