Oh...and I am wondering if the set up in Flash Back is something that will be set aside for Season 3, early storyline.
Barry talks about visiting what we know as Earth 3... so I think that will be an episode next season (a new earth each season, with Earth-SG being a special exception).
Barry has the device,but works on it the rest of the season...perhaps the finale he journeys...but winds up on Earth 3.
Also,not sure if mentioned here or just the Supergirl thread...but they could do a version of Crisis, where at least some earths merge...if they felt they could mix the two shows.
The Crisis would be done mostly offscreen, but we see the results.