Being able to extend native widgets with custom behaviors could help make things much more elegant, though—such as augmenting in-browser autocomplete (to highlight search criteria, etc.) on top of already existing DOM with just a little extra JS, rather than putting together a completely bespoke one from a bunch of divs and a lot of code (counting third-party dependencies).

It would definitely be nice. That being said, I do think it'd make more sense to let people "roll their own" for at least a few years first. Just to get a really good idea of what the problem space is, and what defaults people are likely to expect.

I've seen a lot of projects try to set things up from the beginning without enough information, and it ends up falling a little flat (or worse, getting in the way).