In addition to the laundry list of heroes we could meet, The Legion Of Super-Heroes also represent a team possibly coming to the Supergirl fold that Kara, Superman, and Mon-El have all been a part of at some point in the comics.
It appears Superman has some explaining to do to Kara in Season 3!
The present assumption is that Superman has already had an adventure with the Legionnaires, and keeps the ring as a memento of his past journeys.
More importantly, it opens up the floodgates for a countless group of heroes like Saturn Girl, Lightning Lad, and Cosmic Boy to make appearances in Supergirl.