There are several rumors flying around about the nature and content of the sequel to Man of Steel - many of which I myself am taking with a grain of salt until things become more concrete. One of these many rumors is how Green Lantern will be handled in MOS2, and in the DC Cinematic Universe (DCCU) in general. I personally don't think Green Lantern will appear in MOS2 - there's no need for him to appear, and it would likely be overkill.
A rumor that has appeared on CBM has suggested that Dwayne "The Rock" Johnson could potentially portray the role of John Stewart as GL: http://www.comicbookmovie.com/fansites/notyetamovie/news/?a=92347. Though I don't see The Rock as a good choice for the roll of John Stewart (Idris Elba seems to be the ideal choice for this character in the rumor mill thus far), the decision to use John Stewart in the roll of the Justice League Green Lantern is a good one. Here's why:
1. It promotes diversity:
One of the things that made the Justice League and Justice League Unlimited cartoon series work is the conscientious focus on diversity. Instead of using Hawkman, they went with Hawkgirl; instead of Hal Jordan, they chose John Stewart. Wonder Woman is no longer the token female of the group, and the all-white super hero team is broken up by heroes with brown and green skin. This is good because it makes the Justice League MORE accessible to the audience from numerous angles.
2. It leaves continuity options open:
By aging Ben Affleck's Batman, the creative crew behind MOS2 is leaving the audience with the option of considering the Christopher Nolan movies as part of the new DCCU continuity. It is potentially a good marketing move. In a similar move, using John Stewart in the role of Green Lantern means not having to recast Hal Jordan. Ryan Reynold's outing has been praised and panned. Rather then totally wipe that movie out of existence (a sequel could potentially revitalize that film series), focusing on another GL, and expanding the GL mythology, is a healthy option.
3. It provides a needed personality type:
Ryan Reynold's personality in GL is arguably very close to that of the Flash - witty, cocky and brash. We don't need 2 cocky super heroes taking up screen time. Batman is elusive and paranoid; Superman is commanding and worldly; Wonder Woman is assertive and harsh. Flash's wittiness has to be balanced with GL's stoic commitment to duty.
It is too early to know what is going to come of Man of Steel 2 (perhaps re-dubbed "World's Finest"). But I'm one to hope that MOS2 maintains focus on the relationship that develops between Batman and Superman. We could get a glimpse of characters like Flash and GL, but only in some in-movie news report. Wonder Woman should be a cameo as Diana Prince, who is perhaps revealed in full in the after credits before her own film. John Stewart should be a cop or military character in a Flash movie. Stewart's role is a major supporting one. Perhaps he's the one trying to bring a team together to take down an interplanetary threat - this would make the most sense due to Green Lantern's cosmic roots.
Time will tell the tale...