Many of such initiatives fail because too few people care. If I had to point success criteria it would be: how many people believe the initiative is valuable and are going to get actively involved.
In the first place I'd think about bottom-up than top-down approach. Top management, sooner or later, will stop caring about the idea or will get all hot about another one so people who might keep it running aren't there. They are fellow PMs who want to improve their work.
Now, if you want to have enough fellow folks working on the thing you must answer the question: what's in it for them? As long as they can see value for themselves they'd likely follow you. However if you can hardly answer the question in a reasonable way, that's a huge risk for the initiative.
Note: people rarely look at such things form the perspective of organization. It's not about what the company gets, e.g. improved project management method, but about what I get, e.g. less work, nice achievement in resume, etc.
And one more thing - there should be someone who takes care about dropped balls. Someone who put some effort to coordinate things, fight those small fights whenever needed, pushes people to do something and brings some fresh ideas. A leader. Not a formal initiative owner but someone who feels accountable for keeping the machine running. Otherwise at some point you may find that for a moment everyone is overwhelmed with everyday tasks and suddenly no one will overtake some effort to gather people again to get back on the track.