If the studio is unable to decide the future of Reynolds' hero by the time Justice League actually gets moving, a ringbearer other than Hal could be seen in his stead. Not to launch a sequel, but provide support.
And that, comic fans, is a role clearly made for Guy Gardner: the embodiment of a confident, steak-eating, beer-drinking American male. A stereotype to be sure, but actor Michael Rosenbaum has several roles under his belt - not just Smallville's Lex Luthor - that called for being obnoxious, smart-ass, and yet likable; all qualities needed to make Gardner work.
DC's New 52 version has Gardner as a washed-out cop from a police family, so besides providing some expertise for the League, that backstory would anchor Rosenbaum to an actual character, keeping the wise-cracking (which the Flash voice actor has experience with) from falling into shallow development. That's if Marvel doesn't snag Rosenbaum first.