SQL - understanding how to interact with databases will be an incredibly useful skill for whatever rabbit hole you next want to fall down.

FOSS, learning curve, and niche field? I got you. VLSI layout software or SDR (software defined radio) software.

VLSI layout is kind of software allows you to design electronics hardware at the silicone/transistor level. IOW you have to draw the layers of materials that form transistors (cross poly silicon and p/n-diffused silicon to form) and connect them to metal layers. Learn how NAND and NOR gates are actually formed. Make a 1 bit data storage (flip flop), half adder, full adder. Go as far as you want.

Example software:

• Magic (what I used in school): https://opencircuitdesign.com/magic/
• klayout: https://www.klayout.de/

SDR. Less intense, but also niche and gives you lots of related things to learn/branch in to.

Can be used to receive and decode radio/satellite transmissions. Normally people would buy hardware and use software locally, but some people make their receivers accessible on the internet (websdr) which can give you a first intro to it. After you capture data you can run it through other software to decode (depending on what it is). For example, you can capture raw weather image data from satellites and decode it to get the images that the satellite is transmitting as it’s capturing the data.

A list of some SDRs you can access over the internet (doesn’t require hardware purchase to use): http://websdr.org/

It probably won’t be immediately obvious how to use either SDR or VLSI software but you asked for niche and learning curve, so enjoy. ;)

EDIT: Fixed some typos and grammar.

There’s always software reverse engineering. As bad as it may sound to some people, the US NSA has a github repositroy for a tool known as Ghidra, under Apache 2.0 license, so you can look at the source code and determine whether or not it’s spyware. If it’s actually NSA quality, you know it should be good enough for reverse engineering old software that won’t land you in trouble if found out by the original devs. Last update was 2 months ago.

I tried a little bit but have yet to go very far at all with it since I am an idiot who thinks he’s smarter than he really is.

Though, taking a page from your book, I am interested in looking up rekordbox now.

ssh and scp (secure connection and file transfer)

grep (the standard command-line search tool)

sed (Stream EDitor, good for find-replaces once you learn how to use it)

awk (can operate on text files or command output that’s organized in columns)

sc (terminal-based spreadsheet)

trash (safer replacement for rm that moves things to your desktop’s trash directory instead of deleting immediately)

One week of keybr.com to improve touch typing skills, which should serve you across all your IT endeavours… Unless you choose to learn gimp and drawing stuff :P

Factorio

Ardour, it’s a FOSS Digital Audio Workstation

https://ardour.org/

Sketching 3D in FreeCAD.

https://buttplug.io/

You could get JOSM and become an OpenStreetMap editor. https://josm.openstreetmap.de/

Also install gentoo.

Mmkay thank you, this is such a great thing to ask.

Blender and buy a 3D printer afterwards

Lilypond. Its music software. Learning it will keep you occupied for at least a week.

Emacs

You can do really neat thinga in QGIS – like make your own professional quality maps, make map animated gifs, etc