The piroshki are good either warm or cool - and I really recommend this recipe.
-salty cheese can be added to the potato mixture;
-mushrooms braised with onions and salt and pepper, to taste;
-green onions slightly braised and combined with boiled eggs cut in small pieces;
-braised cabbage with a bit of onion, salt and pepper;
-braised sauerkraut with onions, salt and pepper;
-cheese mix of yellow and soft white cheese (Romanian telemea);
-any minced meat, braised with a bit of onion, salt and pepper;
-cow's milk cheese;
So there is plenty of room for improvisation.