We could use mystcraft to generate two island worlds with limited space and resources that could act as each teams homeworld. Right at the spawn point have one way portals going to their bases so that new people don't have to wander around too much to find their team, but then have portals linking the teams world to two different, separate parts of the overworld, making finding the other teams portal another challenge. Island worlds don't have a lot of room and certainly not much resources, so there would be good reason to have to go out into the overworld for everything you need. Make the realms eternal day and not only would it feel more friendly, but also prevent a lot of mobs spawning, meaning you can only get all those zombie brains and magic levels outside. For the servers sake I would add a "no writing ages" rule. Keeps things from getting too out of hand.
You could also use a server plugin to give build protection over the entire island to the team. That way when nobody else is online somebody can't sneak in and destroy everything in a single night unchallenged. Give both teams access to some traps (Torch Levers is good) and they can effectively defend themselves while offline, and raiding becomes a lot more interesting than just run in and blast shit. You would need an honesty rule that you can't just lock yourself in and use wool blocks for doors that you tear down and put back to get through, but it would make things pretty interesting actually having to deal with any cunning traps set out.
I know Thaumcraft has a door that only opens for you, and that is a little close to impossible turtle defense, but if you used it for anything important it would lock your entire team out, so not that great really.