Currently when PC is leaving the level time on it freezes. When PC comes back all the things remain there unchanged.

Suggesting to change the existing behavior so that several turns are passed when level is loaded (PC comes back).

Exact number of turns I'd suggest is 5-10 for descending/ascending + number of actual turns passed on another level. Limit that number by 150-250 because larger number won't make much difference.
Another nice addition would be bumping up monster HP and PP regeneration rate - they just resting and healing wounds while PC left them alone. It might be a bit extreme but at some point monsers could get corrupted from background at appropriate DLs at these loads.

What's good about it:

• It makes an impression like the level was living even when PC wasn't there. I'd say good impression.
• It removes easy healing exploit "go to another level and wait" since monsters would heal too and regain power.
• It makes stairs less safe place to store loot. Which discourages a lot from taking more items than you can carry. And it finally makes game less annoying and more challenging.
• It makes farming and ratling traders less annoying because herbs would grow, traders loot would restock, etc.
• Leaving compnions/slaves on another level and going for your own business becomes potentially unsafe too unless they're able to handle themselves on their own.

Cons?

• Makes life harder for those who likes exploits.
• Everyone got used to existing behavior.

Probably something like that is already implemented. But I can't notice the effect. Probably it was already suggested. But I can't find RFE. Hence posting it.