I usually just let the role evolve dynamically out of a group. In any particular group of players you have leaders and followers, builders and dreamers, etc etc. I like to simply categorize players between varying scales of "active" and "passive". Active players take the lead, prefer to have the reigns over moving the plot forward, like to add content of their own, etc. Passive players prefer to follow instructions, get suggestions, work in a team, etc.

Active players will naturally begin to fill in the co-GM role for passive players by answering any questions they might have or offering suggestions to them as they learn about your universe. They'll also do this with each other, though due to their nature, might engage in a conflict over who's right, so you gotta keep an eye on that. Other than that? After a few months, it's fairly clear who's inserted themselves into the co-GM role, and so long as you've done a good job nipping problem players in the bud and encouraging the good ones, you will still have the power, but they'll be there to gladly help you with any new players.