Agreed on Kanye.
and for John Stewart..
A number of guys would work. Chiwetel Ejiofor, Chadwick Boseman. Anthony Mackie, Idris Elba, and Dwayne Johnson would all be great but they're already established as other DC or Marvel characters.
I saw someone suggest Terry Crews and I think that could be pretty awesome, especially alongside Nathan Fillion as an older Hal.
If it's not Aaron Paul, then John Krasinksi is my pick for Barry. John Krasinski, along with Aaron Paul, has the potential to lead a big franchise, and his personality meshes well with the way Barry is typically portrayed.
Justice League War wasn't for everyone but it's an easy way to get acquainted with the Justice League members' personalities. John Krasinski fits in with the New 52 portrayal of Barry Allen that was in JL War.
Aaron Paul can pull off the lighter aspects of The Flash, there are moments of levity in Breaking Bad. The dude has the range for the role. He's not very tall, but neither is Tom Cruise. Camera tricks fix that. But if they want to go for a bigger guy, John Krasinski is 6'4, so are Momoa and Affleck.