A cow transformation has potential. We could use a very simple premise: A woman enters a seemingly abandoned barn, and runs into a cow, who licks her hand (or something similar) which could act as a trigger. The next two minutes would depict her gradual transformation into another cow. Besides the physical details of her TF, subtler things could be done to visually show her mental degeneration, which you don't often see in mainstream transformations.
Alternatively, we could use the same scenario, except with anthro-cows instead of full-on cows. Something along the lines of this video, except with a little more effort put into the makeup/prosthetics, might be cool. (Warning: It's creepy as hell.)
https://www.youtube.com/watch?v=cKx-Ko8zWt8
Ducks are my second choice behind cows.
Just so you know: I'm only a writer, so all I could contribute to such a project would be a working script.