Doom I can agree with in regards to up and down aiming being a little redundant, but I definately agree with the others that full mouse aiming is a big part of duke3d's gameplay.
As for Duke3d, in my humble opinion it'd be a worthwhile endevor for an inclined 68k develeoper. It (duke3d) wasnt much heavier on hardware than Doom in thier original pc forms, so following that idea it could be much better than it currently is (even quake2 is faster on 68k than 68k duke3d).
