Another couple of contenders who've been totally neglected so far-
Whatever happened to the Pre-COIE Barry and Kara who went back with Post-COIE Superman, Lois and Parallax to stop COIE.
Superman just says that they set off elsewhere along with Parallax. And I believe Parallax has shown up on New 52/Rebirth earth since then. What happened to Barry and Kara though? Could they possibly be showing up?
Plus, with Rebirth Barry now remembering COIE, seemingly, it does beg the question...
The Bart and Connor idea is great too.