To give more details on this idea I would like to suggest adding NPCs that are either solo and/or NPCs that are in tribes and they're level of hostility is based off your actions.
For instance, a NPC tribe (an eco friendly tribe) in the Red Woods on the Island map are curious but friendly people unless you start being destructive (chopping down trees, aggroing their names) in the vicinity of one of their members.
Or make some NPCs naturally hostile as their self preservation has taken over and they mean to do whatever means necessary to survive on the ark.
For example, a NPC alliance that lives in a tell habitat under water would try to kill you and any underwater mount of yours as they chose this lifestyle to avoid the dangers that dwell on the surface above, and view any survivor as a threat to their existence.
In addition to this, to make it even more challenging to survive in singleplayer, NPC tribes have the ability to do Tribe wars. Well how would you know if an NPC tribe has launched a Tribe war against you? Simple! Listen for the sounds of war drums being played on the back of brontos as they March their way towards your base. Would all NPC tribe wars be the same difficulty? Of course not! NPC tribe wars would become boring after you've survived through the first 5, and become annoying. Instead, make NPC tribe wars difficulty scale based off the level of the single player survivor or the advancement of the NPC tribe.
For instance, if you're to scale off survivor level then a NPC tribe wars at level 50 would be much harder to defend against than a Tribe War with a level 5 survivor as the tames of the NPC tribe would be of a higher level and better bred for war.
If you're to scale off advancement of NPC tribe then things would be different and more random. What do I mean by this? Well, let's just say you'd be better defending against a Tribe War with a beach front NPC alliance with bows and clubs, than you would be in a Tribe war with a NPC alliance who live in underwater tell habitats that have a Plesiosaur with tell turrets on its back.
Lastly, for survivors who want to continue playing without NPCs. Put a toggle true or false option in the game menu for players to select before loading up their saved game or before they start anew.
I'm not sure if this what you mean but it's the first idea that comes to mind when I think of NPC tribes in Ark.