SWTOR had NPC companions from the beginning.
There is another MMO but the name eludes me.

Honestly the NPC group thing would be lovely. As it'd help with a lot of the community's complaints.

1. Folks would no longer be "forced" (I use the term loosely) into old content just because 1 person needed a clear.
2. It would allow people to go through the MSQ without queues & the drama of being the 1st timer in a group of disgruntled veterans.
3. It would cut out a large number of the "Bads" that everyone says are ruining the game and that everyone resents in their groups.
4. Folks could in effect learn fights/dungeons & their class without hurting/hindering/endangering other players.

I'd honestly just every Dungeon & Trial get a Solo mode. Awards Gil, Seals, 50% of the group Tome award, or ONLY award the group Tome reward qty.