Of course, SPOILERS, for both the Flash and Arrow below.
While re-watching season 1 of the Flash with my brother, I came across something I don't think anyone really realized...(I'm probably too late but whatever..)
In 122 (episode 22-Rogue Air) Team Flash decides to work with both Captain Cold and his sister, Golden Glider. They have to move the meta-humans from the pipeline to Lian Yu. They move them by truck all across Central City to Ferris Airfield. Ferris Airfield!
At Ferris Airfield...
Cisco asks where everyone's at, Barry replies with something like so," It was shut down after one of its test pilots went missing."
The Test Pilot...
is of course Hal Jordan. If you are familiar with the comics, or even the not so popular film Green Lantern, Hal (Ryan Reynolds) works there, as a test pilot, flying jets and such.
Arrow season 4 (Episode 1-Green Arrow) gave us a nice cameo of Hal Jordon...well, his jacket at least, in one of Oliver's flashback of his time in Coast City, with Amanda Waller:
What we do not no is, when Hal is going to appear...My guess is probably the last season of Arrow.
Earlier in the Flash season (episode 14-Fallout) when going to save Professor Stein from General Eiling, they figure out that he's being held in area 27. Wells asks if Barry can see if he can find any useful information also, Cisco says something about finding some aliens.
The aliens could be Abin Sur (the alien that gave Hal Jordan the ring), Martian Manhunter, or possibly Super-girl.