Unfortunately, I don't think enough sites have been opening to provide a current "trend".
I have been trying to follow the development of some petsites, but they are usually either so generic that they aren't interesting, or so complex that it takes too long to open and users (and creators) lose interest.
I do see a lot of wolf sims though; that should create tons of competition. I think that is the only "trend" I have seen so far. But as of yet, I haven't see a petsite that has completely blown me away. That's not to say that I'm not optimistic for some to open, but those projects usually lose their headway and never open.