Fenixp: What is there to enhance? Really, Blood is one of the few shooters that I just can't think of anything, it worked as it was. Unless he just wants to add a grenade shortcut.
Navagon: More modern controls. By that I mean things like properly implemented mouse look rather than the clunky shit in the original game.
But yeah, there really isn't a lot about that game you could change without risk of ruining it. I don't even know if HD graphics would be an improvement. The low-res-ness of it all left a lot to the imagination. What style would you use in HD? Torchlight-esque painterly? CoD so called realism? What would work?
Just keep the original "style" and upgrade the resolution. :\