IF YOU HAVEN'T SEEN CLANNAD/CLANNAD AFTER STORY, DON'T READ.

What if the ending of A.S. was just Tomoya dreaming. Like, after losing both of them, his heart literally breaks, and he's just lying there next to his dead daughter, having this dream of "What if?", as he slowly dies. Or maybe the ending is him meeting them in his own version of heaven, where they go play at the flower fields forever.