What I'm posting here is quite B-I-G and can be extramely huge in PvP, but has a strong impact on PvE too.
Not every pet got his specialisation ( water elemental, warlock pets, possibly more ... )
But you can set it and receive bonus hunter skills for the pet! You can make your water elemental charge/dash/heart of the phoenix etc! The passives like 50% dmg increase work too! Imagine a mage with bonus interrupt+stun with charge! Yeah you can have it!
You can see the command necessary for it on the image. The available options are
/run SetSpecialization(1,1)
/run SetSpecialization(2,1)
/run SetSpecialization(3,1)
Tested on mage and warlock. Doesnt work on mindbender/shadowfiend shammy and Xuen. Enjoy!