I know Supergirl is supposedly separate from Flash/Arrow/Legends, but I think it's about as "separate" as Constantine was, wink wink. Anyway assuming Supergirl will indeed cross over with those three shows, then it's already confirmation Superman exists in this universe. It is nice to see some more direct reference to Superman and Batman, hell, they reference Green Lantern all the time and apparently he's off-limits too. And we've seen Kord Industries, etc. You know what would really blow people's minds? Have them travel through time to whatever the hell year Gotham is supposed to be set in.