So you tame a tank pet like a bear. A rune will allow you to take control of the bear while the hunter becomes a ranged dps pet
Why not just let the pet tank? If you want to roleplay as a hunter pet you can play a druid tank.
I like the idea of pet holding threat but not being the main point of control.
I think the hunter should still be the primary thing you play but my idea for hunter tank is:
Rune of Spirit Bond:
50% of damage taken by your pet is transferred to you and 50% of damage taken by you is transferred to your pet.
In addition 50% of healing received by your pet is transferred to you and 50% of healing received by you is transferred to your pet.
The exact numbers can be adjusted to whatever % would be balanced or other tweaks made like adding in some flat damage mitigation if needed based on testing.