One thing I think would be interesting, though it may have already been selected, would be different door types and sizes. Some ideas for a different type of door would be:
curtains hanging on either side for the more mystical or hippie themed group
Sewer grates or flood valves that need to be opened, for bases that are in sewers or underwater themed (think a submarine)
The classic sci fi sliding doors, complete with little "whoosh" sound as it opens and closes
Multi level rooms would be neat as well, complete with stairwells or elevator platforms to take us up to the higher level for those without flight or jumping capability.
It'd also be nice to have some way to display pictures or statues of the members of a super group. This would be a great way to honor those heroes or villains who have not been able to play as much or had to leave the game due to real life circumstances. Our founders in the Shadow Keep have both left the game, or moved on to other servers. It'd be really neat to be able to honor their existance in some way, as our group wouldn't be around, or have as many neat features as it does, if not for their involvement early on. It could have a plaque that would give the hero's ID info and rank within the group on it so that everyone who enters the base could know their history and rank within the group.