Right now we seem to have only two acceptable options: Total destruction or return to status quo ante bellum with very little modifications. How about we add a third?
If I think of the modern period, I can find many historical examples of states that became client states through economic/military domination, however that is outside the scope of BM. Thinking about the medieval period, I have a hard time thinking about any other mechanism than simply oaths.... Is there a specific example you have in mind and would like to see reproduced in BM? From there, it may be easier to work out the mechanism to make it happen.
