XHTML shows what happens when you try to agree on everything upfront. You mention agreeing on a basic syntax. The browser developers generally do discuss ideas for new features in basic terms before they release them, and hopefully there's some vague sense of agreement between at least two browsers before a feature goes alpha. But agreeing on anything more than that, instead of just hammering out a working implementation, means developers end up with APIs that are hard to use and don't address their users' needs.
Like I said before, they are getting better at agreeing on basic syntax at least. I would say don't make it available until the basics have been decided on. If they are still arguing over details that require a prefix then it isn't ready for release.
