Honestly growing up, I didn't really read comics. I didn't have a comic book store growing up near me. So as a kid I grew up and watched the cartoons. Batman, Superman, X-Men, Spider-Man, Justice League and others. I'm not saying my way is better than any of yours but just hear me out on where i'm going with this. Now I grew up on Wally and John and I ask if you please understand and see where I'm viewing from. Let's begin with who should be Flash.
Barry Or Wally As Flash
Now as I said I grew up on Wally but it doesn't matter who you pick between them. The Justice League animated series didn't overly focus on "who Wally is" like it did with Hawkgirl, Martian Manhunter, and others. You really didn't know who Flash was if you think about it. You know Flash isn't the brightest member and is funny but that's really it.
Now while Wally was used, Barry should be used in the Justice League movie. Barry's backstory is in a way even more tragic than Batman's and Barry is very similar in terms of character to Peter Parker. Barry is used in the comics and other media nowadays so use him.
Barry Allen As Flash
John Or Hal As Green Lantern
Honestly, I didn't really know of Hal Jordan before the movie and if you didn't read comics you didn't know him either. The only main cartoon that stared Hal was Superfriends which was ages ago. Sure he had an Green Lantern: First Flight but you didn't know Hal until the Green Lantern movie. When that hit we got the Green Lantern: Emerald Knights, Green Lantern (live action movie), and Green Lantern: The Animated Series. So in a way unless you read comics, up until 2011 you didn't know Hal Jordan.
Now let's look a John. John is one of the most popular Lanterns. He was widely exposed in Justice League: The Animated Series. John also stared in the gam Justice League: Heroes and was going to be played by Common (which I still think is good casting) in Justice League: Mortal.
The point is the public knew and loved John but didn't know Hal. Sure someone could make a point using comics but at the same time, the comic industry in going as downhill as the music industry. In February of this year, there were only 8 Million comics sold. That's Marvel + DC + anyone else. That's not really a lot.
Here's the way I feel: if you want Hal then keep Green Lantern canon and bring back Ryan or use John. There is no reason to not bring back Ryan and not keep Green Lantern canon. It was bad but it wasn't horrible. I would give the film a 52%. It makes no sense to not bring back Ryan if you use Hal.
Here's the problem, Ryan has said he doesn't want to come back as Hal unless a good director and writer is attached to the film.
Also there are news that WB's and Fox want to cast and include some heroes that aren't "white". People who are of other races.
Now the only heroes are are black that WB's would include would be Cyborg or John Stewart. Yes Martian Manhunter but at the same time Martian Manhunter isn't "black". He can shift into any character of any race of any sex. Also there are no rumors of Cyborg in the film. Green Lantern will be.
John Stewart hasn't been a disappointment in the public eye like Hal has. Let's be honest when you think of Hal Jordan in the general audience, you think of how good the animated series was but you still have that bad taste in your mouth of the live action film. John has no bad taste.
The best choice is to bring in John Stewart or if you want Hal, bring back Ryan.
The other problem with Ryan is his ego. There have been rumors of Armie Hammer playing Batman, Gina Carson playing Wonder Woman, and Charlie Hunnam playing Flash. We know Cavil will be Superman. Add Ryan's name to the and he's the one who stills out like a sore thumb. Because of his popularity, he wouldn't fit it with the others. Casting an unknown like Chadwick Bowmen (who is with WB's and plays Jackie Robinson in 42) would fit.
The only problem with Hal not being back is of course, Geoff Johns. You kind of have to lock the doors of the room so he doesn't have any input like he did for Green Lantern. Look how that mess turned out.
Cast John Stewart Or Bring Back Ryan As Hal