I must admit I didn't actually read your post.

Basically what you are asking is anyone can upload a pet.
There would have to be a system to use personality.
There would have to be a system for breeding.
There would have to be a system for affection.
There would have to be a system for actions and mood.

Every-time a pet breeds a new pet would have to be graphically created...
Basically, hypothetically, every two hours, per person, a new pet should be graphically created because that is when they can theoretically be bred.

So lets say 100/1000 people have a pet, in a day 12 new pets per 100 people would have to be created. That is 1200 pets a day. (Since a player creates his own pet, the possibilities of breeds are endless.)

That is, for 100 people, 438,000 pets per year.
Each pet having lets see.. By the minimum.
1 sprite for idle
2 sprites for walk
2 sprites for sleep (getting up / down and sleeping)
2 sprites for drinking (head in the middle, then down)
2 sprites for bathing (transition between two for bathing?)
3 sprites for breeding (woohoo is complicated)
1 sprite for yawning
2 sprites for gralat getting (open mouth slightly, open mouth fully and fire)

That is 15 sprites, which all need four directions. 60 sprites per pet so far...
A sprite is an image.

I'm honestly not so sure, assuming only 100 people will have pets per year, that a graphics team member would want to create 26,280,000 sprites per year.

And I would feel like the systems needed for it would be a waste of time also.. But more possible then the original creating a pet and breeding idea.

However, I'm pretty sure more than 100 people will want a pet...

I hope these statistics give you an idea why this pet system thought should not be created...
I may think a pet system would be an amazing idea,
but I also know what is possible and isn't. :x