Yeah, I also think that the "flash-sideways" characters are remembering their past lives in the main universe. Something happens that resets the world and everyone gets an alternate life. Then they all remember their previous lives and have to band together to... do something.
Makes me wonder if this is the first time this has happened. Maybe they're all going round and round through alternate lives until they get it right. That might explain all of the strange coincidences in the season 1-3 flashbacks.