Maybe a limit to the amount of mansions on each island? (or as suggested high ranks can only own them, they could hire low ranks to build them though
) Hmm... i really like the idea of a basement anyways, you could have a secret trapdoor hidden under you bed! It would be a great addition to a theives home to have his own black market in the basement
You would even have secret groups down there too... *pokes Stefan with the basement plan*