The rumor that Barry accidentally crosses over to an alternate universe, and runs into someone there who's also pretty fast? A guy in red and blue...PLAYED BY TOM WELLING?
Ooops! Looks like someone posted about it below.
1. Given Tom Welling's reluctance to even put on the Superman suit, I highly doubt this will come to pass.
2. Geoff Johns himself is on record stating that Superman, or Batman, will never appear on Arrow or the Flash.
So there you have it.
Except that Warners /DC has the final say on all of this, and they just might give the thumbs up to the idea, if a one shot wonder!