Legion of Super-Heroes, which both Supergirl and Mon-El were members of in the comics continuity. This isn't the first appearance of an LSH flight ring in one of Berlanti's shows and I'm thinking its prominent appearance in this episode—an ep dealing with Mon-El leaving Kara and Earth—is meant as foreshadowing that he will be following his comic counterpart's exit to the future and the Legion. If I had to guess, I'd go with his mother doing something intended to strike at Kara and Mon-El saving her by taking it on himself. What that something might be, I don't know, since the show hasn't used Daxamites' notorious lead allergy yet but whatever it is, it will likely be something fatal and the only cure to be found is in the future, separating him from Kara and this show.
The LSV would be cool, but there's been zero buildup or ground laid for that.
The Legion of Doom? Those dudes are so damn lame I don't want them anywhere near this show. They sully the good name of that group with their awfulness. They just suck so bad.