One thing on Mercs, at least with the ones in Rhys, they don't work in multiplayer last I checked (the multiplayer version of rhys had to remove it) Just a side note if you plan to have multiplay compatibility, that being said, could it be added through advanced diplomacy to 'give' a unit for a predefined number of turns, and then it is returned to the original owner? Then the gold/favors could be done as part of the weight, as long as the ai knows how to use it effectively, it could be a way to do it without adding in a whole new sub system.