The Justice Society has been a major part of Legends of Tomorrow Season 2, though, so maybe Melissa Benoist could drop by Legends of Tomorrow to induct Power Girl into a J.S.A. rebirth. Who knows?
Of course, the Power Girl of the comics is traditionally associated with the Justice Society of America, not the Legion of Super-Heroes.
The second half of Supergirl Season 2 may be introducing the Legion of Super-Heroes of DC Comics fame; maybe Power Girl could be one of the members of the Legion down the road.
There are a few ways that Power Girl could make an Arrow-verse debut that would actually make a lot of sense.