hmmm... since its a different earth and the multiverse is currently closed off

I would use Melissa Benoist as Earth 2 Power Girl in lieu of Supergirl. A Barry Allen (Grant Gustin) was shown to exist on Earth 2 pre-Crisis, but he wasn't a speedster. Does he still exist on Earth 2? These are potential crossovers characters that could appear in a crossover poster without really reopening the Multiverse. If the Flash appeared, I might suggest a flashback to the original JSA Flash. It might be fun to see Grant play Silver Age Flash in Flashback. The current (present day) Barry could be a relative, if he still exists on Earth 2 post Crisis.