I think that in world gen, the various nobles that exist (Lords, i.e. those that control castles or manorial villages, Barons, Counts, Dukes, Kings) should actually have parts of the land under they're direct control. Furthermore, the greater nobles should assign parts of they're demesne to they're subject nobles, with the king granting land to dukes, counts, barons and lords, dukes granting lands to counts, barons, and lords, etc. and also controlling part of they're demesne themselves (even kings should have part of the land under they're direct control). This should apply to humans as well (in fact only humans have lords right now, unless you mod other races to have outdoor fortifications like I do, personally I think dwarves should have some kind of fortification, perhaps mountain halls should combine the functions of human towns and fortresses) so they should have the noble ranks of barons, counts and dukes.
This should have an effect on fortress mode. If, for instance you create a fortress in an area that is controlled by a noble you'd have a chance of attracting the appropriate noble or having him send/appoint from you're population a subordinate noble to rule you're fortress for him. If you're creating a fortress in unclaimed territory, you'd probably have you're land made a barony with a noble appointed from you're population like it is now. Of course when the game gets to the point where you have hill dwarves to rule there might be a possibility of you sending nobles to rule over them.
If you establish a fortress in non-dwarven territory there are three options depending on how much the locals like you. Either they'd try to conquer you (possibly even if you're civilizations aren't normally at war), "offer" to send a administrator to "help" govern you're fortress (possibly turning into option 1 if you refuse), or offering to send a liaison.