Coalitions - Be able to form alliances and power blocks with factions against other coalitions; they were a big part of politics of the period, (E.g. Seven Years War, War of the Austrian Succesion, Napoleon.). They would have to be quite flexible though, as they had a habit of changing quite regularly.

Congresses - Wherin you can get two different coalitions to sit down at a big conference to make peace with each other, and borders are redefined. The person who called it (Not necessarily the AI) would make suggestions as to the drawing of the new borders, and these would either be approved/rejected into a final peace deal. Minor factions would be unable to protest the gobbling up of their lands, although major factions would be able to veto any cessations of their lands, probably trigging a war against the members of the congress who accepted the deal. (Note the difference between rejecting the call for a conference, resulting in a continuation of hostilities, the disapproval of a certain outcome of a congress, resulting in the proposals for the outcome of the congress being rethought [I suggest a 3/4 majority approval of a plan being submitted, for that to result in that proposal being the definite outcome of the congress, and an inability to submit that plan again, to stop any stupid AI behaviour] and rejection of the agreed outcome, resulting in a war between the other factions who accepted the outcome of that particular congress.) Features like bribes and double dealing would play an interesting part.

This is needed because although the diplomacy at the minute works fine when interacting with the player, it seems incapable of responding to threats from other factions, and co-ordinating it's response with friendly factions. Methods such as these could force the AI to work together.