The following comic preview for The Flash depicts a scene from next week's episode, and before she's interuppted, you'll notice that Gideon mentions that Barry Allen is a founding member of The Justice League. Now, it would be easy to dismiss this as a meaningless Easter Egg, but Harrison Wells (a.k.a. Eobard Thawne/Reverse-Flash) has been all about keeping the future in place, despite creating The Flash with the particle accelerator years earlier than he was in his time (explaining why this version of the character is younger than in the comic books). It's obviously all rather confusing!
While Superman and Batman are unlikely to ever appear on The CW, could we one day see The Flash form a small screen Justice League with heroes like The Arrow, Supergirl, Firestorm, and The Atom? It definitely seems like a very real possibility, don't you think? If not, this is still a cool nod...