Superman in the form of Tyler Hoechlin.
Although the initial report on Routh's return suggested that he and Hoechlin would be playing the same character in different time periods, the Arrowverse does already have a much simpler way of introducing Routh's Superman and allowing him to interact with Hoechlin's Big Blue Boy Scout too: the multiverse
This is something the Legends of Tomorrow.
The current Routh's Superman to be a version of the Man of Steel from just about anywhere else in the multiverse.
This makes even more sense considering that the crossover is Supergirl, and Legends of Tomorrow, and it means the door to the multiverse is open even wider than it's ever been before. The Arrowverse crossovers always like to have great fun with the possibilities opened up by alternate universes - such as the heroes fighting evil versions of themselves in the Crisis on Earth-X storyline - so introducing multiple versions of Superman fits right in with that ethos.
Superman plays an important role in the Crisis on Infinite Earths storyline, although it's likely the Arrowverse will be looking to mix things up quite a bit. It's already set up for Green Arrow to die in the event, as opposed to The Flash, and if Routh's Superman ends up taking the death of Supergirl from the comics, then it would mean it can have a big crossover moment while allowing the Arrowverse to have one Superman left in the form of Hoechlin's Clark Kent.
