Oh my gosh, I always longed for a feauture like this! So I'm definitely supporting it. <3
I'm pretty sure I added all the pets I missed to my wishlist, but sometimes I have this doubt and think that, just maybe, I missed a pet somewhere. So it would be neat if we could easily find that out without looking through all these pets in the archive.
I remember that I found out I actually forgot to add a pony quite some time ago. I always assumed I owned all the ponies from that year and was never aware that I actually didn't own one of them. And then, by chance, I found out I didn't own it and I actually missed that pony all that time, for around three years, whithout me ever noticing. xD
So yeah, I really think that this feature would save us from such dilemmas, haha.