Cats & Dogs is already my favorite expansion pack, but your ideas for a refresh (if we ever got one) would make me love it even more than I already do.
I especially love the idea of a Pets Festival, which would honestly fit in perfectly with Brindleton Bay as it is. Perhaps one of the festival events could even be a pet show that your Sim and pet can possibly win prizes for? Kind of like the animal show in Henford-On-Bagley?
Also an additional lifestage and more breeds of both cats and dogs would be a big yes! I would also like to see there be some more whiskers and tail options. Two of my kitties have eyebrow whiskers in real life and I'd love to be able to have that option in game without relying on CC.
And definitely more toys for our pets, especially ones that our Sims can use to play with their pets.
I love all of the ideas people have suggested here, but these are the ones that stuck out most to me!