That's how my friends and I play Civ. We had an issue in the beginning with players getting knocked out in the first day of a game that might go for 5 or more play sessions, so we devised a scheme where a player could be forced into vassalage. They can no longer win outright, they merely exist to support the conqueror. The conqueror gets to overrule any decisions made by the vassal, but there's a bit of a gentleman's agreement that this power should be used sparingly, since it's annoying to be micromanaged, and it slows the game to a crawl. In particularly aggro games, the same vassal might be handed back and forth between two players still in the fight several times, because as a relatively underdeveloped nation, they represent a fair peace settlement.