Hello Phil,
What I say is, additional to official roadmap (who should be decided by a board [ellislab and others community leaders]) developers from the rest of the community can request new features. The board can disscuss what of these new features are important and relevant to add officialy to the project, make plans public and then any developer can implement this ideas, of course following a sets of rules.
All this is apart from other submited code by the community members( patches, additions, plugins etc.)
Seeing these roadmap/plans as a complement to GitHub milestones because on this (https://github.com/EllisLab/CodeIgniter/issues) I can see any kind of issues and at least for me is a little confusing to know what new features expect on the next stable release of codeigniter appart from bug fixes.
My intention is to have a clean and clear vision about the path that follows the framework.
I think that making something like this gives certainty to the project and makes the community more involved with a positive impact on the framework getting better and robust through an increased community complicity in official releases.
Anyway thank you for your answers and I also apologize for my lack of expertise in the English language.