The RFC process sounds very nice but be careful. PHP has been doing it for a while now and while it's working out nicely for them in some regards - pretty much every php person I talked to involved with it hates 'internals' with a passion.
We do development of new features behind feature flags, and have automated tools to help enable, disable and strip them. We're planning on moving to a model where we do an approximately annual "garbage collection" pass on the framework (corresponding with a major version bump) and simply nuke all of the code that has been deprecated since the last major version release.
There is a long history behind the hating of 'internals' in PHP land. The RFC process has improved matters, but can't get rid of all the poison that already existed.
