This gives me a good idea for teaching my kids to program. Why start from scratch? I'm going to give them a game like this, with code, and see if they can do something crazy with it!
P.S. For me, I became the computer lab teacher's assistant in order to skip recesses or other activities in middle school. Fixed printers, made .bat file menus, good times.
That Quake 2 and later required Visual-C++ and such kinda took the spark out of the creative mods, as the entry price became too high.
For instance Quake 1 had a "flight sim" of sorts that was basically done by one guy using Quake-C and some simple, freeware, modeling tools.
There was an attempt at making a successor for it in Quake 2, but even with a larger team etc it basically ran out of steam thanks to the complexities involved etc.