I like the idea,but maybe it could be its own submod as well for people that just would like additional monuments?
Castle Dawnguard would be good, maybe give lessened professionalism cost for hiring mercs.
Ysgramor's Tomb could also work, as well as the 7 pilgrimage sites from morrowind.
Some of the non windhelm hold capitals could get their castles as well, would be very flavorful for a skyrim conquest.