I'm currently most excited about the Incremental Static Generation RFC, particularly with its fallback behavior.
Besides the feature giving you a fully incrementally generated static site (powered by any source, including headless CMS or direct database calls), the fallback behavior helps this scale to millions of pages.
With the fallback: true behavior, you can tell Next.js to only statically generate your most popular product pages for example. Then, at runtime, Next.js will lazily statically generate the remaining product pages when the first request comes in—that lazily generated page will then be used for successive requests.
This allows you to keep build times low, and still allows for incremental rebuilds once deployed.
I'm also excited about the new Environment Variables support, which should greatly simplify env handling in Next.js apps.
If you want to know how direct database work in Next.js' SSG support, checkout this cool tool we built to see how Next.js bundles code for the client-side.