Huge Spoilers ahead! Read at your own risk!
If you're a comic books fan, you might as well had a nice time watching these TV shows. For the first time in history, we see multiverse in digital screen. There are certain characters that might come to these series or they may have their own shows in the future.
Power girl is Supergirl's doppelganger from an alternate earth. You might know her from Infinite earths comic series. After she comes to Earth 1, she joins the Justice League. If there are infinite earths, why can't there be a power girl?!
During the Golden Age of DC comics, before Justice League, there was a group called Justice Society of America. After DC created the Justice League, they moved these guys to a place called Earth-2. Just like comics, we saw many of the justice society in The Flash, Arrow, and Legends of Tomorrow. We saw Wildcat in Arrow. He was the who taught Laurel how to fight. Then we saw Jay Garrick in season 2 of Flash. Hawkgirl and atom are both in Legends of Tomorrow but, sadly, Hawkman died. Knowing that, they'll either change Legends of Tomorrow characters or they continue with the old the team.
This is a hard one to catch, but if you're a comic book nerd, you may have heard the dialogue between Winn Schott (Jeremy Jordan) and The Flash (Grant Gustin) in Supergirl/Flash crossover.
"One [Earth] where all of us are evil."
"Been there, it sucks!"
Crime Syndicate is a group of supervillians from Earth 3 that each member shows the exact counterpart of Justice League members. Instead of Superman, there is Ultraman. Instead of Batman, there is Owlman. and etc...
Superman, Batman, Wonder Woman, or Green Lantern
When flash travelled to Earth-2, he went to his doppelganger's house and we saw their telephone. It's not clear how a non-metahuman would know 3 of the Justice League primary members but they DID show us that he knows them.
Considering the fact hat Patrick J. is joining Legends of Tomorrow, he can either be Booster Gold or Green Lantern.
Not enough evidence? Rip Hunter speaks of Superman and Batman in Legends of Tomorrow
"I've seen Men of Steel die and Dark Knights fall" he said
Superman is already in Supergirl's world but can he, also, be in the Flash universe?