The first thing that came to mind when I saw the question is perhaps a bit of a weird answer--but I really want to learn SELinux. It's completely overkill for my Linux desktop and the few services I run on my network. The same with OpenLDAP, I want to play around with it even though I have no real need for it with my setup, I just haven't gotten around to it yet.
On that note, I also feel like I want to learn Ansible, or some other configuration management tool. The thing is, I haven't even played around with it (or any others) enough to really even get what the intended use case is. I'm looking for ways to manage policies and configurations across multiple machines in a common way, but it feels like the more common use case is deploying webapps. So while it's on my list of things I want to learn I don't even have sufficient background at the moment.
Then, finally, the other thing that came to mind was timeshift--or really BTRFS snapshots in general. It would be nice to have that additional feeling of safety while playing around with my systems.
Seconding Sway. I will admit I prefer autotiling (switching the split for new windows between horizontal/vertical automatically, rather than choosing which split you want), but overall Sway is so good in configuration that I still use it in spite of being a manual. The configuration takes time, but that's common to pretty much any tiler.