I personally have nothing against high res texture (I have produced enough of them over the years) but and I mean a big BUT, you have to consider the bigger picture.
If you upgrade something with better textures then the rest of the game has to match as well. I like to call it the
Domino Effect and it has to do with texture density not resolution. The key to good texture work is maintaining texture density across all assets.
As necros said, the best upgrade to Quake would be to free all assets from the 256 colour palette and finally have a rich colour environment.