I can definitely tell you what apps NOT to serve- cheese and crackers. I just went to a small unassuming pizza party where cheese and crackers were served beforehand. Between the cheese in the apps and the cheese on the pizzas- it was just too much.
TBH, I'm not 100% certain that I even support the idea of apps with pizza. Pizza, done right, is just so glorious that it really doesn't need a warm up act. Let's face it, anything you serve first isn't going to taste as good, and, no matter how light you go, it will, to an extent, lessen the appetite and detract from the pizza experience.
I guess, if you start with wine, you're obligated to serve something with it. Whatever you choose, just make sure it isn't filling and that people can't toss back too many of them. Puff pastry, perhaps.
It's tough. By it's nature, pizza is a bit of a starch fest, so, in theory, a meat app would provide some nice balance, but something as heavy as meat is the last thing you want to serve. Personally, if I were attending a party, I would be happy as a clam with a meat app served with the pizza, not before it, so you could get the full glory of the pizza on a relatively empty stomach, but still have a little protein on the side. Buffalo wings would be nice, but I'm pretty lowbrow like that

I guess, in theory, pizza need not be such a starch fest, but being such a traditional plain or (thinly sliced) pepperoni kind of guy, it's tough for me to experiment with more meatier options.