We're not going to shock anyone by casting Idris Elba as John Stewart, since the British actor has distinguished himself as one of the most underrated leading men currently working, period. With size and skills to convey both physical threat and the intelligence of a US marine-turned-architect, there's no other we would more trust with the role.
Elba is currently appearing as Heimdall in the Thor series over at Marvel, and may end up getting his wish of portraying Luke Cage with the studio at some point. But if Marvel deems Cage a poor fit alongside bigger heroes, and Thor 2 is as open to character deaths as rumored, he could be available to don the green ring.
Beyond just bringing in the under-20 crowd due to Stewart's role in the animated Justice League, Elba is too good for either Marvel or DC to pass up; in our opinion, we'd rather see him rescue Green Lantern and do Stewart justice.