The total war games had bi directional treaties and they were very easy to exploit. The player in EU3 could rort it so badly. Just annul a half dozen treaties and demand a crap load in return. It's not historical, but it is practical and done for game purposes.
People more or less already do that in actual EU3 diplomacy.
Matter of fact such comportment is SO predictible that it could be modded out in a second (just like the swapping of empires or crappy exchange)
EU3 ALREADY has a value system created for its treaties, each province has a different value based on culture, core, CB, etc... Same with ducats.
I don't see why we couldn't exploit that system to allow bilateral exchanges or peace treaties. (whereas if we had to code in all the values for the exchanges it would be impossible)
As I said earlier, if games like the Total war series could do it, (and i think we all agree their diplomacy is puny compared to EU's) surely we can.
I liked the idea of negative ducats (which would also allow the long overdue "Purchase of province").
The way I see it, the "Offer tribute" and "Demand Tribute" Tabs could be merged onto a single screen.
As such (sorry for the size)