I am actually against NPCs; I think it's part of H&H's charm that the entire world is emergent from player behavior. There are two things, however, which must be addressed:
1: A way to guard locations 24hours a day.
2: A way to trade while offline.
I suggest that both of these can be fixed by allowing PCs to remain "semi-active". You could log off at a guard post and then use a simple A.I. to respond to threats. Since you'd be using your normal stats, you couldn't even use an alt to cheat out of that...
Similarly, you could log off at trade stand and allow other players to make offers of a list of goods you have. Then when you get back on, you could review the offers and make trades you liked. In fact, you could even set some offers to be automatically accepted, like "30q Loaf of bread: 2 q20 Ccoins" or "50 Q50 gold coins: 5 silk threads". It'd be really nice to be able to walk into town with some money and just buy things.