They have yet to have their own DC Rebirth. They've been out of the spotlight for two years now and haven't headlined their own comic since 2013. The Legion of Super-Heroes is a beloved superteam with almost 60 years of history (they debuted in 1958), and it's time for them to return.
What better way to kick off the 60th anniversary party than with a return to live-action television?
Yes, the Legion did have a special appearance on Smallville (not to mention some animated appearances), for three of the members at least, but now it's time to go all-out. At San Diego Comic-Con, producers of the various Arrowverse series didn't say much about this year's crossover, except to say that Supergirl will be a full participant this year, making it an actual four-episode crossover (along with The Flash, Arrow, and Legends of Tomorrow), instead of having only the final moments of the episode tying in.
That means it's time for the Legion.
The Legion of Super-Heroes is a massive team of powered teens/young adults in the 31st century. Inspired by the heroes of the 20th (and 21st) century, they band together to reveal corruption in the inter-planetary government and promote good over evil in the far-flung future. Occasionally, various members have traveled to the past or brought heroes from the past to their present for help in classic team-up fashion, and that's what we need to see this fall.
The Legion of Super-Heroes has been teased on two of the shows already. On Supergirl, a Legion flight ring was on display in the Fortress of Solitude, which seemed to imply Superman in that world had spent some time in the future with the Legion as Superboy, as he did in various comic book stories. A similar Legion ring was seen in The Flash when Barry, Cisco, and Wells went to Earth-2 and saw other universes in their travel. Legends of Tomorrow hasn't directly referenced the Legion, but they are a time-traveling crew, and Rip Hunter is the son of Booster Gold, who himself has a (stolen) Legion ring.
Supergirl, of course, also brought in Mon-El, though they did so with major changes to his story. In the comics, Mon-El is a member of the Legion (and Mon-El isn't his real name; he takes it on to honor Superman and the House of El, but that's neither here nor there). The end of Season 2 saw Mon-El exiled into space, where he entered a mysterious wormhole. It's the perfect set-up to bring in the Legion: You simply have Mon-El zapped through both time and space, and the mission in the crossover is to return him to the 21st century (now wearing a Legion ring to protect him from the lead in Earth's atmosphere. Boom).
A Legion of Super-Heroes crossover would put Supergirl front and center, would offer up a more concrete reason for the Legends to be involved, and would give The Flash a look at what an optimistic, efficient group of heroes can accomplish. Green Arrow, um, would be grumpy and annoyed, probably, but that's when he's at his best in these crossovers (as the de facto "Batman" of the group). It would pay off the teases, give us a good way to bring Mon-El back into the fold (and pay off his Legion link), and, most importantly, bring the Legion of Super-Heroes into the public consciousness just ahead of their 60th anniversary next year. Heck, if all goes well, it could even lead to a viable expansion of the Arrowverse (dare we hope/pray/beg for a spin-off?).
CW, Arrowverse producers, it's time for the Legion of Super-Heroes. Take us to the 31st century. Long Live the Legion!