So it would be nice to be able to buy and sell villagers at the market instead of having to kill the villagers to make room for more units. Or gift the villagers to other people.
For that matter, it would be nice to be able to gift units to other people. Or request units or even buildings from other people. Under the diplomacy tab, you should be able to click request structure, and send a request to transfer the structure to you to the ai, or to another player.