The "Arrow," "The Flash," Supergirl" and "Legends of Tomorrow" crossover ended with the Dominators leaving Earth. The heroes succeeded in driving them away, although they should not count on them not returning.
There is a difference between scaring the Dominators away vs. completely annihilating them. The Flash, team Arrow, Supergirl and the Legends only succeeded in scaring them away. They did not kill the aliens, when the crossover ended with "Legends of Tomorrow" Season 2 Episode 7, "Invasion."
This being said, there is a likely chance that the Dominators can return to Earth. When they do return, they may be more prepared this time to face the superheroes.
Furthermore, Executive producer Marc Guggenheim teased the return of the aliens in a recent interview with TV Insider. He confirmed the Dominators will wreck another Earth, this time in "Supergirl." The aliens will appear later in the season.
However, when they do return to Earth 1, then it would not be the first for The Flash, Green Arrow and the Legends. On the other hand, it will be the first time the people will encounter the extra-terrestrials. In the "Arrow," "The Flash" and "Legends of Tomorrow" crossover, the people only saw the ships and not the Dominators.
"The way we're sort of playing it going forward is that Glasses is good at his job, and when he says he's going to cover it up, he's going to cover it up," Guggenheim said about the cover-up of the aliens in the crossover.
Another clue that teased the return of the Dominators in either "Arrow," "The Flash" or "Legends of Tomorrow" is Barry Allen's message to Rip Hunter. Future Barry told Rip about an oncoming war, which Guggenheim said relates to Flashpoint. Is he referring to a war created by the Dominators on Earth 1? After all, the aliens were part of the ramifications of Flashpoint.
