Potentially a good idea, but restrictions would have to be included/mentioned, and allowing one to do this treaty could make any flaws in the trading system highlighted significantly. One example is the limited number of provinces that one can trade during a given game, this could probably be abused. Maybe there could be restrictions on when this is allowed until a better solution is found? One potential allowed time is when the 2 nations would have to have been at war for 5 days straight with each other and have gained/lost at least 6 territories in that time period. Some of these may not be very ideal, but you will have to recognize, this game is about war, not necessarily peace. For the most part, you will have to fight this enemy, unless you want a coalition between the 2 of you. The most beneficial places for these treaties would be the larger maps, where these wars can escalate to much higher levels (50 player and higher). It's not ideal on a 4-player map because you should seriously be able to figure these kinds of relations out with your 3 enemies. If this is added, the game would need to be marked that treaties are allowed on the map. Just some other things to think about, but nice idea.
"War does not determine who is right - only who is left."