@Kyronix please do not make this more complicated than it needs to be. A revamp would work very smoothly by allowing customization of what's currently available. Combat skills + magery where applicable + resists and elemental damage. Don't take this to a level where it shouldn't go by adding in stats, HPI, DCI or HCI.
Example: I have a dread mare that is 3 slots. That would leave me 2 slots to increase. I would increase fire resistance and most likely wrestling to bring it up to a 5 slot. We also should have the ability to swap special moves whenever and within reason at a cost. This would make some lower end pets better while not taking away the pecking order of creatures in the game. A greater dragon should be the king of pets but a cu sidhe should also be worth using.
But please feel free to add in a rideable pet that will spam Holy Fist at 4/6 casting for me. I would enjoy this in pvp.