I recently started building up a starter theme called Tango, which attempts to bring everything we love and expect from Underscores, into a React-based and API-driven environment. Ultimately, I’d like it to become a sort of library of its own, that could be dropped into existing Underscores-based themes and handle as much (or as little) as a theme developer wants on the front-end.
Tango also relies on a little plugin called WP-API Theming, which fills some holes the current API lacks at this point when it comes to theme development. Anything theme-related specific to the API I’m just dealing with in there for now.
The coming API (and the modern development tools it allows us to use) is going to usher in an exciting new generation of WordPress themes, and I can’t wait to see what people do with it.