The original post was a little mixed up about what I mean by a 'Core' role, but here is a clearer explanation:
Core NPCs are the NPCs who are not dependent on a venue type, but appear in your neighbourhood no matter what. These include the Postal Worker, Repairman, Maid, and Pizza delivery person. These are the NPCs I envision appearing in a 'Neighbourhood Management' menu.
Lot-based NPCs are the NPCs who come with specific venues. I imagine that when you place a Library, you would be prompted to hire an existing Sim as a librarian, customize your own librarian, or allow the game to generate a random librarian. This would be very similar to the Hero system in Sims Medieval, with the added option of choosing a pre-existing Sim to fill the role. This goes for any type of lot - Gardeners and Fishermen for parks, Food Critics for restaurants, Rangers for national parks, Reflexologists for spas... the list goes on and on.
Your Sim could work as a core NPC or lot-based NPC; either way, that Sim treats the NPC job as a rabbit hole career, and leaves your household from 9 to 5 every day. When you're playing another Sim, you would be able to see this Sim in action, whether coming to fix your toilet, giving your Sim a massage, or judging your culinary creations.
