Ooo, sorry 'bout the...uh..."V" word.
I'd just like to develop my own engine (with a cousin actually). I don't know much about making an engine, and since the Quake 1 engine is open source, why not? As I develop the better engine I'll learn a lot about engine programming. Stuff on hierarchies and BSP stuff (Quake 1 uses BSP, right?) Either way, I'm a fan of Quake 2's solid game design.
I'm not using an enhanced engine for some of the reasons above. Now, I'll probably look and see how they made their modifications (no, I'm not copying) but I won't use them. It's sort of a "It's my baby and I programmed it" situation...