I hope they have pets. It would be really great to include pets.
The question regarding pets isn't so much an "if" question as it is a "when" question. Both the expansions for Sims 1 and 2 were incredibly popular (although I fail to see why), so you can bet that it will happen at some point before there's a Sims 4.
Pets in Sims 1 were beyond annoying. It took forever to train them, so getting yours to the point to where it could do something pet related (which was pretty much
just pet shows) was somewhat close to impossible. The only thing I liked about it was that new part of town that came with it. I haven't heard all that much that was good about the pets expansion for Sims 2. The people who didn't like it stated clearly why, but anyone who
did like it couldn't say much more than "OMG PETS!!! They're so cute!" Not exactly a glowing...or even comprehensible review if you ask me.
Sims 3, on the other hand, I would likely buy a pets expansion for. Even if I don't like the pets themselves, EA has a tendency to toss in a bunch of stuff that will appeal to everyone.