Superman makes sort of a guest appearance on *Supergirl". Green Lantern was kind of written off when Flash went to Ferris Aircraft, which was shut down, and Barry knew about ir, saying "One of their test pilots disappeared."
So that's what happened to Ryan Reynolds' Green Lantern - he "got disappeared". That happens, y'know.
Nobody has mentioned Wonder Woman. I should think it was inevitable that WW would come out of the shadows to investigate the doings of metahumans in Central City, and when Supergirl is seen, this becomes almost mandatory. I think it would be great if Gal Gadot could be contracted to a couple of appearances, say mid-season. That would place it perfectly for the release of BvS. Both Arrow and Flash and Legends need crossovers to make all the stories work. This season's Arrow has Damian Darhk (somebody needs to be kicked hard in the ankles for that spelling) and the appearance of magic. Flash started mid-story, and the team is just discovering that metahumans exist that were not created by the explosion. Flash has to break off Captain Cold, Heat Wave and likely Golden Glider (I hate that name) to Legends. The time line mess has to be explained, hence Rip Hunter. New costume. New Zoom. New Wells. New Iris.
Which reminds me: the Deathblow character (Doug Jones) was killed by Cold. I wonder if there will be an autopsy, a la Zod, as Deathblow was the key that revealed pre-explosion metahumans.