As well as new special effects, and the film remastered to high def, I would also love (and I'm dreaming, now) if they scrapped the synthesised music from seasons 1 and 2, and just replaced it with selected orchestral music from the latter seasons. I think it would make the episodes seem so much better.
I was with you until that bit. Don't change the music - it's part of the atmosphere of the episodes as much as Edward Brown's lighting. I much prefer some of those Ron Jones scores to the later bland Chattaway durges.
The original 35mm film before special effects, probably exists in storage. This original film is NOT in 4:3. Yes, it may have props, lighting, or other stuff in the frame on the sides. But this kind of stuff can be erased and fixed with VFX software.
The trouble is, that's just making more work for yourself, most of which wouldn't be seen due to overscan.