4. Mekhi Phifer (as John Stewart)
Suppose someone over at the WB/DC offices was very smart. They would recognize that while Hal Jordan is the most popular and enduring character in the green suit in the long run, Stewart is the more familiar character to kids, young adults, and comic book readers (their target audiences). He was the representative Green Lantern on both the "Justice League" and "Justice League Unlimited" cartoon series, running consecutively from 2001 to 2006. He's also been the lead Green Lantern character in the comics since the New 52 relaunch in 2011.
So, while my idea to use Stewart as the rookie sounds clever and exciting to me, the smart money would be to cast someone older and make Stewart the experienced protector of Sector 2814. In fact, the most recent comics have assigned him the role of instructor for new Corps recruits, which would be fun.
If the movies went in that direction, Mekhi Phifer would be perfect. He's tough as nails, but highly intelligent. He would do great with Stewart's penchant for bucking authority in exchange for common sense decisions. And his natural delivery of dialogue would certainly go a long way to help the meta-franchise root itself in the real world, as opposed to an alternate dimension where everyone speaks in biblically-charged symbolic monologues delivered in hushed reverent tones.