As one of the persons reached out to for that effort (and I'm not a shiny contributor), my current unpublished work will eventually serve as a basis for some proposals to shiny for review that maybe after some testing and input from others will stand on its on merits, for such a proposal.
Even as a side gig for people, I think shiny is pretty far along; but I think shiny has reached a critical point where every decision may get cemented and people want those decisions to be the best possible ones.
It wouldn't be entirely difficult, just time-consuming, to pump out all those widgets by cementing in a bunch of other decisions; but then we just sort of have this Go version of every other GUI toolkit with the same every-other problems. There's a good opportunity to research new ideas now.
Shiny is already doing and demonstrating a lot of important things; but event and configuration, and a single unified layout and animation step are two things I think we could all be doing better.