My idea so far: Writing a clicker game, where you have to write code. Each click is a line of code and by buying upgrades (hard- and software, food) you can increase your output and write better and faster code.

As I'm using a C64 (or at least CBMprgStudio and an emulator) and I will use BASIC, the theme might be fitting more than I like. Running out of RAM space could become a problem. ;)