I like any idea that allows for more pet food - with more buildings to feed, I am always running short.
I saw the different suggestions for amounts of pet food per week. Keep in mind that evolving buildings are on mostly one day, with a few 2 day production cycles. Then a few of those have a feeding effect that produces more if we use pet food. How many times a day, max, do we have the option of producing pet food in the magic academy? Would it be best to have an evolving building that continually produced pet food, one that produced pet food when "fed" raw materials (for example, 10 days of food production for X spell fragments and/or Y broken shards?) Whatever the amount, you could balance it by saying 1 max accrued until picked up, forcing people to not get the max amount unless picked up every day.
And though I like the animal collecting food idea, I'm also kind of partial to the idea of a goblin pet food factory, or some such.
Too bad the buildings can't affect what appears outside the city limits, you could actually have us looking for berry bushes to pick or something.
It has a LOT of potential, and would effect people's willingness to place buildings which require feeding in their cities. I, for one, do not place as many for the simple reason that I cannot feed them all.