I think NPCs are a great idea. I think that as you build your guild claims you can attract more NPCs. The NPCs you attract depend on what buildings you have available and what professions (and skill level) are available in the guild. For instance having a blacksmith shop will attract a blacksmith NPC that will work the forges and can generate items (at a slow rate) and can even have them for sale for people who for instance aren't black smiths but need the item. The item and its' quality depends on the number and level of blacksmiths in the guild as what items you stock in the black smith shop.
The algorithm for attracting NPCs can include the number of active guild mates and time spent in game, housing available, food available, furniture and decorations in the claim, if the roads are paved getting to the claim and in the claim. There can also be rare NPCs that maybe only stay in an area for a small period of time before moving to another area. Also to keep your NPCs you need to keep conditions up in the claim.
Another aspect would then be guard NPCs. So you build a barracks and then can generate a certain number of guards depending on the level of your guild, quality of buildings, equipment made available (stock the barracks with weapons and such). You can set the guards to walk the town claim and keep people from stealing stuff. Can also set them to gaud the gates in/out of the claim etc.
Can have hunter/gatherer NPCs that work the fields or hunt animals. For gatherers the Windmill and stock house could be used to attract them. Hunters would patrol the area with weapons provided by the guild and hunt local animals for food/leather or can tend coops/barns/stables and again sell their wares.
Building a keep or town hall will allow the guild leader and his "advisors" (or whoever he gives permission to) to command the claims NPCs. Certain NPCs may make comments when you interact with them and them may say something like thy want/need warm clothes or would like more decorations around the town and players can fulfill these to keep the town NPCs happy etc.
Finally there can be raiding party NPCs that will raid personal claims and guilds either at random or based on some algorithm. This will test your defenses and ability to protect your claim.
Typed more than I intended.