Guy Gardner's place is in a Justice League International movie or as a supporting character in a Green Lantern Corps movie.
I also agree Elba would be absolutely perfect for John Stewart. If they have to use John Stewart, and Dwayne Johnson is Black Adam or Shazam, then it's gotta go to Idris Elba. He's such a great actor, that he'd make me okay with Hal not being involved. If they get Dwayne Johnson or Idris Elba, and the writing for the character is as quality as it was on Justice League TAS, then John Stewart will work well.
I'm getting used to the idea of John Stewart being the movie Green Lantern. The dynamic between John and Wally was pretty great on JL TAS. And the Shayera romance was actually really well done... i wouldn't mind seeing that in the movies..
Man JL TAS really was amazing. I get why people want John Stewart. Some of the best Green Lantern media EVER is from that show. In Blackest Night and Hearts and Minds are my favorite Green Lantern centric episodes. Both are amazing.
A criticism I have of the Justice League animated series was its handling of Sinestro. He was a recurring character on the show but there was zero ties between John and Sinestro on the show. Sinestro is one of DC's best villains, and they realllly dropped the ball with him on that show.
If John is used, he should have Hal's relationship with Sinestro. Sinestro would be a huge asset to the DC movie universe, and I'd rather not see him at all if his relationship with Green Lantern is similar to their relationship on Justice League TAS. I also think if John is used, he should receive the ring from Abin Sur instead of Hal.
The key to making any Green Lantern work is good writing and casting a good lead. It doesn't matter what people think of GL before JL comes out, same thing with Aquaman. Everyone thinks he's a joke, but then they cast Momoa, and all that's going to change. It's all about the actor and the writing. Green Lantern's movie damaged Green Lanterns in general, not just Hal. But that won't matter once the JL movie comes out.