Greater desktop integration, so that it’s possible to share a file by using the context menu (right click > share with Nextcloud). At the moment, as far as I know, the only way to share a file is using the Nextcloud web interface. Also, some kind of file browser built-in to the client to achieve the same thing would be useful.
What operating system are you using. Normally, it should work already.
I use ownCloud so wasn’t aware that this was a feature. Have you got documentation for this? How does the menu supply the sharing settings? Is the file automatically copied to a particular location on your Nextcloud storage?