Discovery of arbitrary code execution hacks in vanilla DOOM games lead to the creation of ACE Engine.
You can now customize classic DOS version of DOOM 2 with GZDoom-like levels of additional features, that all work just by loading a WAD file like any other.
Check out the completely INSANE demo:
https://youtu.be/h7h7rHDOf7c?si=QsmrP7V4i2J5tO28