Ok, Vegan pierogi. Fillings: Lightly steam broccoli, add shredded cheddar cheese. Or steam cauliflower, add breadcrumbs toasted in butter, and some lemon zest. Or mashed potato, gorganzola or bleu cheese, chopped raw red onion, and toasted almonds?
Careful - vegan means no animal products. You described vegetarian options (and very good ones at that!)
For vegan you have to make the pierogi dough without eggs, as well as keep all cheese, butter, etc. out of the filling. My favorite is sauteed sauerkraut and onion (to make it dry), folded into mashed potatoes made with potato, vegan margarine, and a bit of the cooking water. All seasoned with salt/pepper to taste.
Another good one is pesto sauce made without the parmesan cheese (use either capers, extra salt, or nutritional yeast) stirred into mashed potatoes. These are excellent with sauteed tomatoes or spaghetti sauce. If you are not vegan, use parmesan.
My final favorite is mashed potato with finely diced water chestnuts and scallions. I fry them until crisp, then make a vegetarian stir fry with brown sauce (search the web- there are tons of them) - I fold in the pierogies just before serving.
Great ideas here all!