It would be great if we could update custom or paid plugins that aren’t available on the WP Repository. Some ways it could be done:
A) Ask us for the .zip file every time we need to update a plugin (less efficient).
B) We could upload it once (on the Hub) and then it would find it automatically (recommended).
C) Create your own repository from people willing to upload the plugins. For example, every time there is a new release of Elementor Pro I could upload it and then every user could update the plugins. I know it sounds like a security nightmare but you could fix it too (for example only vetted users could upload or you could calculate checksum once you have multiple uploads of the same version).