I can think of pierogi z mięsem, z serem, ruskie(radzieckie), i chyba z kapustą i grzybami. If you want to make Polish food, make Polish food.
pierogi z soczewicą - lentil
z wiśniami - cherry
z jagodami - blueberry
z truskawkami - strawberry
z jabłkami - apple
z kaszą gryczaną i serem - buckwheat and cheese (some fresh mint may be added, making for another variety)
z podrobami - offal
z serem na słodko - sweetened cheese
just off the top of my head, all very Polish and perfectly legitimate.
I've had superb egg and onions pierogi once in a pierogarnia in Kraków.
I mean, our ancestors put inside whatever was available to them, why shouldn't people nowadays do the same? You won't even find a single "proper" dough recipe - some people use eggs, some don't, some add milk or even sour cream, some stick with water, lobbying for it to be cold, lukewarm or hot, depending on what their grandma used.
Though I admit it's hard to imagine pizza pierogi... does pizza dough go inside too? :D