Ignoring time travel, however, you can also get into the concept of Dimensional Analog Characters, like that Injustice comic series had. Black Canary and Green Arrow were in love with each other. In one world, Black Canary died, and in another, Green Arrow died. Doctor Fate then plucks the living Black Canary out of one world, and plonks her down in the world where there is a living Green Arrow. This can get... remarkably squicky. Your true love died? Just realm hop until you find a copy of them and repeat. I swear I remember a fanfiction where the protagonist basically made a harem of his own wife this way. And there's always the problem that no matter how many dimensional analogs you bring back with you, you're never actually saving the one you wanted to live in the first place.