I think this is a very good idea.
1) I would rather have the BfA system tho, the Legion system was much more involved with all the troop upgrading things.
2) 100%
3) It could also add the new Trading Post currency plus it would be really nice to earn some kind of "epic mission" from various parts of the world (maybe tied to expansion?) where it could give you mounts even.
They could also add ways to find new "followers" or "troops" whatever via achievements or doing specific things in the world.
They could all have different abilities and maybe offer unique missions upon being unlocked.
The whole system could be an evergreen pet battle type thing but in mission table style.