I don't think much could be done to top Curse of the Demon. The best thing you could do is get rid of the goofy monster close-ups that were added later against Tourneur's edit. That would basically make it into a perfect movie.
I mentioned before that one movie that would benefit from a higher budget is The Stone Tapes. Great screenplay, great performances, but kind of a shoddy BBC production. It could be redone better. Along those lines, I think the Quatermass movies would benefit tremendously from a bigger budget. They are effects heavy movies made in a time when there wasn't much energy put into visual effects. The ideas, characters, and dialogue are still golden though. Also I would love to see what some sharp young director could do with the Dr. Mabuse series, I think that has potential to be completely badass. One more thing, I think the Blob could be made into a pretty menacing monster. It was always treated in a campy manner, but it could be a real nightmare in the hands of competent film makers. For a taste of how the Blob could be truly horrifying, check out
this old episode of the radio play 'Lights Out'. Take that play, and give it to some serious film makers with a 50 mill budget and that could potentially be amazing.