If anyone is interested in new projects for N64 hardware, Kaze Emanuar has an amazing series about optimizing Super Mario 64 to run incredibly complex romhacks on original hardware.
Interestingly, SM64 is usually throttled by memory speed. This means that using 'inline' is detrimental to performance, since making the program larger can cause another read from memory.
Here's an overview video, he has many more specific videos about specific topics:
Romhacks are typically modifying the compiled binary ROM image. Kaze' work is based on the painstakingly disassembled code from the n64decomp project[1]. He's working in C, modifying the game and compiling it again for the original hardware. Not sure I'd call that a "romhack".
Great videos though!