(TLDR; This is a super naive list, just a few quick thoughts)
I was thinking that perhaps having some kind of % coverage for things that people will need in practice in order to use Elm for rapidly creating nice GUIs. However, I also think there needs to be some kind of (mental, at a minimum) framework for people to work inside so that new controls are easily extensible, styling agnostic, work uniformly with other form controls + validation etc before it can really become mainstream. I'll start adding some thoughts here over time (not much time now) - however, this doesn't cover the second point and I think people still need to get into that before things can really progress very far.