There are already some monuments in SS, a few more couldn't hurt. However, the way settlements look on the battlemap is independent of the buildings in the settlement. In order to have custom settlements(on the battlemap), the settlement has to be created. It also has to be a huge settlement at the beginning of the campaign otherwise if that settlement is upgraded all the changes will be gone. If the settlement is not huge then every settlement that upgrades to huge in that culture will have that custom settlement. So the only custom settlements would be maybe Jerusalem, Constantinople, Baghdad; cities that might start as huge cities in the late campaign but its a stretch.