I've played Book II through five times now and while I agree with many of the current wish list items, I feel strongly about only a couple, so I'll just list those:
- More monsters
In my humble opinion, this is a big drawback. The lack of regeneration (I'd like that) combined with the lack of monsters makes the game seem sparse and underpopulated. I could be wrong, but when designing maps it should be a simple matter to add a few more mobs to empty areas.
- More item variety
By the third game I had memorized every item and combination of attributes, meaning there was no more thrill of discovery and hope for unusual or different combinations of attributes to weapons, armor, rings, and amulets.
These two things, by themselves, would make the game fuller, richer, and more compelling to play. It would also help to negate the need to farm experience in order to develop a powerful character.