One thing I would love to see is React router style routing. I know some people love the file based routing, but for complex web apps I just end up having to hack my own routing onto catch all routes. For example, there is no way to build sub-routers without doing it myself. (Imagine an account dashboard with a common area, but additional tabs for other sub-pages)

Also, a global getServerSideProps ran on all pages would be great. I use getInitialProps in _app.tsx now to make a server side request to get initial user info (e.g. the current user name, access level, etc), but this method is ran on both client and server requests so it has some hacky stuff like returning early if window is undefined or if the requested url ends in .json (when next tries to load a new page on the client side)