React supporting selective hydration would potentially be a total game changer.

I have recently started using gatsby-plugin-no-javascript. It's a very crude version by removing hydration at the page level. It works pretty well for me because my site is a ton of static pages and then one very interactive app page. I get to build it all in React, get a great developer experience, and then all those static pages are like 20k total (css, images, html, everything) and load in the browser instantaneously.

If we could get this ability, but be able to apply it at a finer granularity than pages, we could probably do some really great things.