Right now you can only liberate a city the turn you take it. But sometimes you need to use that territory to finish whatever war you're in, but you don't want that city permanently. Sometimes things change in the future. Sure, you can give cities away, but you don't get the diplomacy boost from that like you do with liberating a city. Would be nice to have that option for more than one turn. 'Hey, there was a war, it's been finished for awhile now, I'm in possession of one of your cities, now that things have calmed down I'm returning it to you'. BIG diplo boost. Would be nice.