Priorities for Customizer development in 4.3:
1) Customizer Concurrency (aka Control Locking)
2) Partial Refresh
3) Menu Customizer
4) Theme Installation
5) UI/UX changes
Customizer Concurrency doesn’t need much Core changes to implement (except for temp hooks maybe), so it makes sense to remain as a plugin, living here: https://github.com/xwp/wp-customize-widgets-plus
@westonruter aims to have beta ready by end of this week.
Partial Refresh is available as a feature plugin and can be tried out now: https://wordpress.org/plugins/customize-partial-refresh/
There are a couple issues still needed for that one and feedback on the API for opt-in.
Menu Customizer will be picked back up to have that ready ASAP for merge review (probably needs about a month). @voldemortensen, @valendesigns, @westonruter, and @celloexpressions will all work on dev, as well as anyone else who wants to jump in. @sheri will get started with design review, possibly with help from others (@folletto). The plugin currently works with WordPress 4.1 or 4.2. We can push lots of testing to start happening now since it’s pretty much functionally complete. It just needs a lot of back-end improvements.
Theme Installation and UI/UX changes were not discussed in more detail and will be worked on as time allows after completing the first three items.