(Proceed with caution... Flash and Arrow Season Finale spoilers lie ahead)
Now as you know, unless you've been living under a rock, the producer of the Flash and Arrow is bringing Supergirl to the Silver Screen. There has also been many nods and easter eggs to Green Lantern in the season finales of Flash and Arrow. But could Hal Jordan tie together these two DCTV universes? It makes sense.
The Key Factors
Green Lantern is heavily focused in space (at least the nodded to in Flash) and aliens. Supergirl will seemingly have a ton of aliens appearing in it. Also, one of the characters in Supergirl becomes Cyborg Superman in the comics. Cyborg Superman destroys, completely and utterly, Coast City: Green Lantern's home town.
Now this could easily happen in Supergirl and have Coast City's destruction mentioned while the Flash or Arrow deal with their own huge threats. Green Lantern could also show up in both shows with the same actor and the CW and CBS don't even have to split a profit from a crossover.