Like many above, I'd like to play a D1, D2, type of game.
You don't need corridors for making it scary. Big rooms are great as well.

The D3 secret test level with 100 monters proves it would be awesome.

I wouldn't mind a true remake of everything official D1 and D2 (D1, Thy Flesh Consummed, D2, Master Levels, Final Doom, No Rest for the Living. Not just graphical enhancement. Games still look beautiful and they don't need them. Real deal, with jumping, mouse freelook. Of course levels would need to be altered a bit to compensate for jumping.

You can use Zdoom and Doomsday Engine to play games based on the DOOM engine with mouselook, higher-resolution graphics, and other things. You can also utilize DOOM 64 EX to play DOOM 64 on the PC with mouselook, which is awesome. I just wish that GOG could make a deal with Id or Zenimax to sell Id's games and to bundle them with these source ports. It would be an coup if GOG did that for DOOM64, because Steam and other online distributors haven't bothered to do anything with that game, and I think it is the closest we got to a genuine DOOOM III.

I liked Doom 64. The fan-made PC port, that is... I can't imagine playing that game on the 64.

The DOOM64 EX project basically allows you to rip the genuine version of Doom 64 and to play it on the PC. This is different from Doom: Absolution, which was made from the ground up. The guy who made Absolution is the mastermind behind D64 EX.

Let's Play DOOM 64 EX
http://www.youtube.com/watch?v=KC2j1lIv5P4