What I like seeing but rarely do is when a fluffy is psychologically abused to the point where it believes it’s irredeemably awful, and realizes just how shit it is, purely because it’s a fluffy, as well as getting fluffies to push past their own programming, like Your Own Advocate and Tiny Lives. The long-con psychological fuckery is really good.
As far as over-done themes, anything babbeh-fever motivated is way up there. As well as the things fluffies get in trouble for in general. It’s always either bad poopies or running away because the owner said no to something like once. I’m pretty sure they can get themselves into far more complex and creative forms of trouble than that. And also exploring the Why can be interesting too. For example, the fluffy in my story keeps getting in trouble for bad poopies, but the Why turns out to be a mental complex around shitting, where she got the wrong message as a foal and believes that ALL poopies are bad, and pooping MAKES her bad, and tries to Stop Shitting (which is impossible for a fluffy) but it just builds up and backfires instead. We get a look at the massive amount of self loathing she experiences just for a normal bodily function.