One thing I kind of miss is autohotkeys on windows. It was relatively easy to do things like set keyboard keys to act as mouse keys. I did that once when I was getting over tendonitis.
These days I have a keyboard with mouse keys on it and a trackball also with mouse keys. I can use the middle button on the trackball and scroll with it, but I can't use the middle button on the keyboard and scroll with the trackball, which would be more ergonomic for me. Haven't figured that one out yet.
That said, I mostly don't miss GUI stuff. I use a tiling window manager and command line utilities to do most things on my system. Its kind of primitive I guess, but the benefit is it works exactly the same on remote systems, headless servers, etc.
I'll add to this that when I first registered at mastodon.social, my default view was some fancied up multi-column monstrosity. I managed to turn most of that off, but it was super confusing at first. Start simple and let users add complexity.
Also agree about the empty feed. At least default to the explore tab. Way better would be a tool to help build your feed based on interests. There are various tools for that, but a user shouldn't have to leave the app and google around to discover them.
A tool to read your phone contacts and search for corresponding mastodon accounts would be intrusive (some would even say unprincipled), but many users would welcome it.