Maybe something underground, just like the houses have, but with a hidden trigger and on a different level. Something about being able to find armories, labs and the sort with mid-to-end game gear. Or just make the hidden trigger ridiculously hard to find/make it not appear 100% of the time/make difficult(but reasonable) requirements for it to work, like a stat check or skill check(example: calling in a perception check while examining a bookcase to find a book or button that opens the way underground.)
Also, some kind of theme for the mansion would be sweet. I would love to barge in a mansion that actually have vampires living in it.