IMHO getting into the world of malware analysis and reverse engineering could be profitable for both your brain and your pocket. And they force you to go deep from day one.
I do not have the expertise to work on either field but this is something in the plan. The good part is that most malwares are targeting Windows so you get a lot of samples.
Also, Windows is an excellent way to learn systems programming because the documentation and tooling is so good and things hardly ever change.
There is a wealth of documentation on MSDN for writing device drivers and such. And great tools for remote debugging so you can set up a VM in hyper-v and step through the code from the host system.
Do make sure if you're analyzing malware you do it in a VM on a machine you don't care about having to wipe, and isolate it from the rest of your network.
I do not have the expertise to work on either field but this is something in the plan. The good part is that most malwares are targeting Windows so you get a lot of samples.