I think the most likely out of the far fetched ones is Diggle as GL, especially with all the teases as others have posted.
My top one is Batman though. I’d nerd out hard if it happened but more likely that pigs will fly before Batman appears proper on TV.
Dig as gl would be nice and more screen time for Superman and batwoman
BLACKLIGHTNING
I really want to see Chris O’Donnell come back so he can be Nightwing and David Mazouz so we can see more of him as Batman or maybe he can play a version of Damian Wayne.
Dietrich Bader as Batman (tribute to Lewis Wilson/Robert Lowery serials of the 1940s)
A Nightwing and a Wally West from an alt universe so we can have both Wallys (if Lonsdale decides to come back for it, of course)
With Conroy on board as an older Bruce Wayne, how about Mark Hamill as an older Joker? 
Smallville (Tom Welling) Superman with perhaps some of the other heroes from that show as well.
Having a shot of Gal’s Diana would be incredible!
Smallville’s Supergirl would be really nice, too as well as if Helen Slater returns to her own Supergirl role.
I’d love to have the last 5 Lois in some capacity (Tulloch for sure but also Adams, Durance, Bosworth, and Hatcher).
You’re all missing the obvious choice. Shaq as Steel.
Hal Jordan - seems like a lost opportunity for Ollie and Barry’s best friend not to show up in the flesh before Arrow leaves the airwaves.
Anything Green Lantern. I’d just love WB to do something with the Green Lantern franchise. It would make an incredible series but would cost millions to do it right. It’s not in the CW’s budget.
Most likely going to be John diggle as Green lantern. They would be stupid not to do it after they teased it. Plus Green lanterns need some screen time outside of that one bad movie and two seconds in justice league.
Hopefully it happens Green lanterns are loaded with content too if they can somehow manage to afford a full show
Could be pretty great
It looks like you are throwing in every super man except George Reeves and Christopher Reeves but who knows you may even did them up I did notice George Reeves costume on DC daily l wonder will power girl be in it pregnant as she was in the original(I think Hal was the father)and if Supergirl has to die let itone of the others I remember matrix Linda Lee cir el and power girl and now Kara danvers
Dan DiDio as himself! lol
Dan DiDio as himself! lol
The original Laurel Lance. I love her E2 version, but it’s time to bring E1 Laurel back. Or, you know, how about actually having Green Arrow and Black Canary show up instead of the lousy OC’s on the show?
Ambush Bug popping up would be great.
Wildstorm Universe characters popping up would be awesome too. Cameos from some of those characters would definitely be “the kitchen sink and then some” moments.
