Hi, I've always been fascinated by the idea of writing game engines..and I wanna see myself in the future as an engine programmer. I'm currently 19, in my second year of undegrads(CS), and with 3.5 years of C++ programming experience, on my way on reaching the intermediate level. After going through quite a few threads related go game engines, I've decided to first learn directX, create a small FPS using that for graphics, then bring all my code together and using it, frame my own engine. After that, learn python for scripting, and learn a suitable physics library (Looking at physx and bullet) for physics in-game.
The thing is, I'm not really into making (only)games, I'm more into the low level subsystems that come together to make that game.. so what should I build for my portfolio which would show off my engine programming skills? Are tech-demos okay? Or should I do something else? If yes, what should it be?