I cannot imagine it will ever happen but I'd like to see the spec precisely defined in terms of core "axiomatic" functionality (JS, layout engine, core CSS rules), and peripheral functionality built on the core functionality.
The core functionality would have a precise (as possible) and extensive definition with an agreed test suite encoding the expected behaviour (as much as possible).
This would allow development of a shared implementation of the peripheral layer, while browser innovation could continue on the core functionality (JS and rendering performance, battery usage etc), and on innovations in the UI.