I know this idea has probably been ran into the ground so much that it's reached the Earth's core, but I think that if Asriel did come back it would be because Frisk gave him half of their soul. That's most likely a very dangerous thing to do, so with the thought that Frisk is nice enough to split their own soul in half to bring back Asriel is really heartwarming to me. If Undertale had a True Pacifist EX run, this is what would happen.