Yes, we already have the companion system. SO using that as a base model, it wouldn't be that far of a stretch to create a job that revolves around and expands on its premise. I would make it a melee job that utilizes whip(like in previous games) and shared gear with NIN and use a pretty standard combo rotation, and its OGCDs could mainly be buffs to your "pet". Have the Job quests revolve around collecting beasts and limit the number of beasts you can get. Maybe have like 3 or 4 outside of starting beast that you can collect and can choose to use. I would personally go for the beasts that are very iconic in FF such as a mini malboro or Cactaur. Or you could potentially create a "glamour" system for the beast, where its 1 beast that you can make appear to be a select number of looks. While creating a potential item catagory for the online store, like the character clothing sets. Granted I dont know the coding side of things, but given that theres already a functioning base model in game and several examples of MMO's utilizing the concept effectively, it stands to reason that the job is a viable and functional option.