Can't believe I'm actually coming to this thread with a legit idea...
This is less of a hack idea than a game project idea, but I just remembered the classic DOS game Sopwith, which I used to love back in my younger days. I've been hacking NES games for a little bit, and the idea of making a game myself from scratch seems interesting to me.
Here's the game for those whose are unfamiliar:https://youtu.be/hBmRUg74EBw
It's a relatively simple game, and what's more, the source code has apparently been released by the original author. The game has already had source ports for modern systems, but I can't help getting interested in the idea of making a NES port which is as accurate as possible to the DOS original, while still adding some graphical and audio enhancements.
Now I just need to figure out how to start. Is it just nesdev and a 6502 compiler?