So, based on the thread about canine companions, a suggestion was raised which I think deserves its own thread.
NPC companions that level up.
I feel it will make the game feel a lot more Fallout if you have NPC backup that aren't just fixed HP hired/enslaved drones.
So I suggest slaves, dogs and a new 'Traveller' type of NPC companion can level up if they fight alongside you.
These travellers show up randomly in bars or in encounters. If you pass a speech/charisma roll you can recruit them for free or a bit of cash.
If they die, they come back to life after a few hours. Same with dogs and the slaves you are attempting to 'train'.
At first, they're terrible in combat, but if they kill enough enemies they 'level up'. Gaining a HP and skill boost in their chosen fields. Eventually they become a decent fighter in their own right, and with a bit more personality than mercs.
They can't be told to follow anyone else, nor can they be told to stand guard. If you want to get rid of them, you have to dismiss them forever. This is to prevent people farming them as free base guards.