You have a pretty limited scope if you think thats the only way they can implement beast master. In some iterations of beastmaster they keep the animals in magic cages and release them for individual attacks, expanding on this you could have a class that every gcd and off gcd is a different beast that you unleash for that attack, that ways you can have as many beasts as you have attacks. I believe only ffxi used the approach of beasts being more permanent pets.