I remember reading somewhere, though I can't be sure where so I'm not 100% sure it's official, that a person's patronus is the same as the animal they'd turn into if they were an animagus. So Harry's animal form would be a stag and Sirius' patronus would be a dog. We also know that if someone wants to enough they can change their patronus. Would doing this also change their animagus form? Or is becoming an animagus sort of like hard-wiring your patronus so it cannot change.
Now, throw metamorphagi into the mix. If a metamorphagus becomes an animagus also, can they change their animal form in a similar way as they can their human one? Would they be able to entirely change species, or perhaps just colouring and proportion? So if Tonks' animagus form was, say, a border collie, would she be able to change into a basset or a wolf or perhaps even a bear or a horse?
What would be Remus' patronus? Can werewolves be animagi?
Here's a thought, what if Nagini wasn't an animal, but an animagus?
That's enough rambling for now I think, feel free to share opinions.