I'm also thinking theres a good chance the upcoming Cw crossover will merge the super, and arrow universes just like the comic book crisis. If I'm not mistaken it's even being called "crisis".
We saw Superman in SG so hopefully Batman, or at least a Bat family member will show up. (I'm rooting for Nightwing, or Batgirl) Supergirl, and Batgirl were BFFs in pre-crisis. I think she might show up. A earth merge would benefit all the shows in my opinion. That could open the door for even some Bat Characters showing up in Arrow. Red Hood would make a nice guest star, or addition to Ollie's team. And Who doesn't want to see the Flash/Superman annual race? Mark Hamill is the joker on Supergirls earth, and his counterpart in the arrowverse is the Trickster. So after the crisis he justs becomes the Joker. So Flash, Arrow, and a Bat person, ( if not Batman) team up to stop him. (Next year's crossover?)