I could see TPTB using the Superman show to do a backdoor pilot for GL. I immediately thought of that when Diggle says that he is moving to Metropolis. What a coincidence! They could easily do an episode where John Diggle's GL teams up with Superman on something. And if it works well, launching a GL series.
Well, the GLs are a big group of people. Just because we are getting one group of GLs doesn't mean we aren't getting all of them. The COIE implied that the new show will take place on another Earth, but there's no need for that, given how much of the story could take place off world. Plus, there are GLs on Earth.
If Diggle is essentially John Stewart, and I like that idea, then the simple answer is to have John Stewart as the GL on Earth, while Hal is off world.
GL could be on Superman what J'onn is on Supergirl.
I do feel that Diggle as a GL would fit in well with Superman. Superman is mainly a solo act, but that doesn't mean he can't form a friendship with Diggle. Even if Superman works alone, there can be other heroes in Metropolis.
Superman has absolutely worked with GLs before. In the cartoon, they did a bit of a hybrid of Kyle Rayner and Hal Jordan's origin story. Superman served as a mentor to Kyle in that episode before Kyle went off to learn from the Guardians.
In the comics, I remember Superman being offered a GL ring of his own, but he turned it down, saying it would be too much power for one person to have.
Diggle/Stewart, as developed, would make a very good friend to Superman.