The Pets EP was seriously empty. I feel like most other EPs had two main things that they added (e.g. Seasons: Weather AND gardening, AL: Apartments AND magic, NL: nightclubs AND cars, OFB: businesses AND subhoods) but Pets was just...pets. They didn't even include a neighbourhood with it and the only theme was the atomic theme. I also remember reading that basically all the code to support pets was in there from the base game and so they literally just...turned it on. I don't think this is 100% true, because stuff like items being pet-compatible or not is a thing. But still.
Sims Unleashed had pets and gardening for example. Now I do agree that gardening fits better in Seasons, so I'm happy that stayed there, but they could have done something different with pets.
Magic, perhaps? In order to have more integration with the spectral cats - there could be all different kinds of familiars - owls, frogs, rats, snakes, spiders? Though, I suppose, that they would have had to skip werewolves in this case and I do think werewolves fit best in Pets. (Also, I already argued for Magic to be the supernatural included in uni!)
I agree with others that there could have been more smaller pets. It would have been fun to have more interaction with small pets, too, like being able to move them between different cages or upgrade the cage, like you can in TS3. The tiny hamster cage looks cruel through today's eyes, and it would be fun to have a choice of a smaller, more basic cage vs a larger one or one with lots of tubes etc to crawl through. In general I love the look and the interactions of the TS2 dogs and cats, more so than other sims games.
I'm not personally bothered about horses and I think the animations probably would be a bit glitchy with TS2 sims and using them as a vehicle unsatisfactory. Plus, I don't think TS2 lots are really big enough for large animals like horses, cows, sheep etc, so I'm OK with skipping them.
Having more general wildlife added would have been cool. Like the bug hunting and birdwatching interactions, from Freetime. I love the mod that lets cats catch roaches, and think this should be in the Pets EP. Likewise there is a mod that lets dogs chase burglars away, which I think should also be vanilla - this should depend on the personality of the dog though - aggressive, genius, hyper, independent dogs should be the best at scaring away burglars and might even pin them down and bark until the sim wakes up and calls the police, but a doofus, friendly dog should try to greet them like a friend instead, a lazy dog wouldn't bother to react and a cowardly dog could whimper and hide. Training could also play into this too - playful (more likely to play) vs hostile (more likely to scare/pin), calm (more likely to correctly identify burglar/ignore) vs disruptive (more likely to bark or play), and relationship with the humans in the household.
I find it a bit annoying that the Pets EP adds a pet noise emitter to every single house - so even if nobody in the neighbourhood owns a dog, you'll hear dogs barking in the background of every lot. I'd make this more random.
I agree that agility training and pet shows should have been added, just ported exactly from Pet Stories. I have the agility items from Pet Stories, but they don't work properly.
Maybe just for something completely different, there could have been some of the stuff from the Family Fun stuff pack rolled into Pets, with all the costumes for kids - I love Phaenoh's "costume trunk" object and think that throwing a costume party could have been added in. Maybe the playground equipment from AL, or smaller, garden like versions - there could have been some fun cat animations added to these! Many people with pets have access to a garden, so it would have been nice to get some items to use in the garden, like a swinging bench, a bird bath/bird feeding table, which would attract wild birds (and cats/dogs could scare them away), a paddling pool for toddlers/kids to play in, adults to cool down in, and dogs can drink from, and jump and play in too, but cats would be wary of (maybe touching the water with a paw and running away, or having a drink). Dogs able to interact with swimming pools! And an outdoor tub for washing the dog - there's a CC one which I use a lot, which is really useful for households that have tiny awkward bathrooms. Actually, some of the
Pet Stories items like the ice sculpture table, buffet table and fancy BBQ would fit nicely into this theme too. I'd like that auto pet feeder and porta-potty, too!
Some adult careers would have been nice- Oceanography fits well here. Maybe a vet career, or more generic "animal care" with a poop cleaner at the entry level, vet type jobs at the mid range, moving onto zookeeper and eventually Dr. Doolittle. Show Business, because it matches the pet career and would have fit that premade family who were supposedly sitcom stars, better.
Edited to add: I love the ideas about pets waking up their owners!
And the idea of a neighbourhood to visit for dog walks would be awesome - this could be a kind of destination subhood, sort of like the Bon Voyage ones, with different rural themes - beach (if you have BV), woodland, meadow, farmland. These could maybe even contain the bigger animals but as sort of hood deco viewable from the lot, so they don't seem penned in, and aren't directly interactable. They could be a "special" community lot type by inviting fewer visitors than usual community lots. And if you have residential lots in the rural subhood, you lack certain features (like delivery groceries/pizza) and the police would take longer to arrive, hence the need to have your own guard dog.
With the rural theme, this could add loads of build mode content too to recreate old style houses like thatched cottages and stone houses.
Oooh and then they could add buses - because villages are often too small to be served by train or major highways, but (at least in Europe) there are often bus routes. So your sims could choose to take the bus somewhere instead of walk to / taxi.
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