The distinguishing thing about pierogi is the filling. People eat pierogi for the filling and not the dough. So, it is important that you pay a great you with attention to your fillings.
Pierogi fillings are broken down into two general classes. They are the savory fillings and the sweet fillings.
The savory fillings includes that ever popular Russian pierogi potato, cheese and onion filling, cabbage and mushrooms, cheese, meat fillings and fish fillings.
The sweet fillings typically are made from blueberries, cherries, and strawberries. But there are many many more and they are also listed at this link pierogi.
When making a sweet filling you will often have a problem with the filling being rather wet. You can avoid this by eliminating the sugar from the mix. Replace it with stevia. Stevia does not draw the sugar from the fruit the way sugar does. And then add some potato starch, cornstarch or arrowroot to thicken the filling.
It is important that the filling be stiff. This applies to both the savory as well as a sweet fillings. You cannot have them leaking liquids or else the pierogi seal will break.
Additionally, if the filling is step you can better control the amount of filling that you put into the pierogi and you can better distributed within the pierogi.
When you make fillings, make them a day or two before you make the pierogi. This avoids the problem of running between the filling and the pierogi. It makes it so much easier to finish cook and serve the pierogi.
When you have leftovers, look at them thinking about how you can turn them into a pierogi filling. Leftover potatoes can be mixed with cheeses, meats, fish and vegetables to make fine pierogi.
Leftover meats when turned into a pierogi filling suddenly transform themselves from being leftovers to a succulent savory filling.
Why this happens and a lot more about what happens to those when you add other ingredients is covered in this book