As the world reaches it's final days, you reach out and search in a desperate attempt to find others to aid your cause in saving the planet, surprisingly enough, you find a group of people who are as determined as you are, they have chosen to follow you and your plan, to protect the planet and thrive.

The Idea :

Whenever someone begins a single-player game, they have the ability to choose if they want to add NPCs or not, as the game progresses they can begin to form their own small settlements and cities, and can be traded with, once the player interacts with an NPC (who doesn't belong to a specific settlement) they can choose to "employ" the NPC, after doing so it will open a menu that will show that NPCs skills and profession, along with tasks to give the NPC such as "Harvest _____ and Bring to _____" or "Drive ______ to ______ and pickup ______ then drive to ______" along with this could also add new "sections" or "divisions" which allow the player to name parts of their land claims, and what they are meant for (e.g. Bradley Division - Mining | Bradley Sector - Mining.) to help the player both organize and assign tasks to the NPCs, the player can also choose to start with some NPC companions (like 3-4), who each will have random low-tier professions, and traits instead of having to search for NPCs off the bat (regardless of if they start with NPCs they should be able to employ more). NPCs should also have needs to be met, and vary depending on their profession, personal preferences, and depending on how late/early game the profession is their needs will vary | e.g. an indoor laborer will need to have a more protein based diet, and a place to rest (chair or bed) prefers coffee and Air Conditioning, an Engineer needs comfortable living area (more luxurious furniture, and bedding) prefers entertainment and alcohol (NPCs will work less efficiently unless one or more preferred requirements are met)

NPCs should also be able "learn" new professions, but they will have to do that job for a certain amount of time, beginning said job, they will do it much slower, but the longer they do it the quicker they can work.

Traits:

different NPCs can also have different traits, some negative some positive, | e.g. a Negative NPC trait could have a condition that makes them need to eat more, require larger living areas, or needing luxurious foods, while some positive NPC traits could let them be minimalistic and not need much do survive except food and a bed, they could be a quick learner ; being able to learn new professions quicker, or a workaholic and work much more efficiently.

Responsibility:

Each NPC will have a Reliability stat (among others like hunger, happiness and conditions), that will allow them to have their own little asperations, such as building a butcher shop, or grocer market, or hunting for their fellow citizens, however (dependent on how high or low said "reliability" stat is) they will have much more unique needs or can generate more / less needs over time.

Sickness:

and as the world deteriorates around you, and your people, many can fall ill, due to the poor quality of air, worsened living conditions, or just the change of season, they will work slower, and will eventually die if not properly treated by a medical professional (another NPC)

NPC Survival:

C'mon, you need protein, vitamins, fibers, carbohydrates, and sleep, the NPCs do too.

Professions:

... Do I even need to explain? you have a niche you're proficient with right? so do the NPCs.

Reason for My suggestion:

although I may be biased because I don't like multiplayer games (let alone run them) but I think it could make the game much more interesting, and more engaging while playing solo, allowing solo players to focus on different things, or just have the most immersive city building/survival experience ever to exist (I mean c'mon, this game already meets most requirements for that), not only that but can add much more possibilities for multiplayer as well.