I think one of the goals this year for customizer will be for there to be a unification of the things you can do in the editor (for post content) vs the things you can do in the customizer (outside of post content). My hope is that this proposed next generation of widgets, JS Widgets, can serve as a foundation for powering a common Content Blocks interface that can appear as shortcodes in post content or as widgets in the sidebar. Nav menu items should also be unified with widgets so that whether you have a nav menu location, a dynamic sidebar, or post content—all of these areas are just containers for Content Blocks. This being the case, I think that Content Blocks is an excellent are of overlap between the editor and the customizer which can be worked on concurrently. I think the Shortcake plugin provides an excellent starting point for Content Blocks in the editor, and I want to start integrating JS Widgets as Post Elements so that widgets can seamlessly be used in post content.