The big bad for the upcoming Supergirl / Flash / Arrow / Legends of Tomorrow crossover is going to be the Dominators, the aliens from the Invasion comic sereis who want to eliminate the threat of metahumans. How will our heroes take to the idea of aliens? Danielle Panabaker who plays Caitlin Snow who is gaining the powers of Killer Frost, told EW.com:
The biggest fear for Caitlin and for Team Flash is how powerful [The Dominators] are and how much of an unknown they are. We're only getting used to dealing with metahumans with these powers that have generally come from the particle accelerator explosion, or Doctor Alchemy, and those are formulas we're a little more used to. Having these guys come in from who knows where, the biggest question mark for all of them is starting to figure out how they're going to battle them.
Seems like everyone will be reacting to the aliens like how John Diggle reacts to metahumans.