Marc Guggenheim has stated that plans for next year's crossover are now set, but no further details have been revealed. However, he previously said:
The one thing we’re all in agreement on—cast, crew executives, you name it—is that we’re not going to go as big next year. In fact, Mark [Pedowitz] and I were talking about ‘returning to the roots’ of our crossovers, and I’ll leave it to people to interpret what that means.
Is there another arc from the comics, or an original Arrowverse story, that you feel would bring our heroes back down to Earth(-Prime)?