hey all great to see some positive feedback! 
I think this could be a basis for a lot of great stories. You are at a long standing war with a certain player, and suddenly this event comes up, and you both decide in your best interests to work together on this one thing, and the long war ends! This directly could add a lot of dynamic changes to every game.
Lots of great ideas here btw, just thought this one to be something that could really change some game dynamics.
There are concepts of magic shards etc... what if a shard was about to die out, but if you and player 3 each give 20 resources of X, your mages together can repair the damage... Now you own this shard now, but perhaps the other player really wants it, and if its gone, no one gets it...