John Stewart as GL could work without even full reboot. Just like in the comics he would just be another GL.
All of this of course depends on popularity. If a JLA is not successful it would probably prevent the lesser known characters getting a movie again for years, if ever. Superman and Batman would not be effected much by a bad JLA movie.
I agree this should start with a World's Finest movie. Seeing them together is what most people really care about. Superman and Batman will overshadow all the other members in a JLA movie anyway.