Try binary reversing microcontrollers if you studied ASM at any point in life it will be pretty eez for you.
You can get some really cool usecases and even "make up" new vulnerabilities and detect them for microcontrollers.
They typically use unsafe languages like C and generally the security from Hardware end is pretty weak as well.
Good luck trying to find anything new on web dev or general OSes like Linux cause they are already researched upon and its pretty unlikely for a newbie to find something significant.