Yes! Different types of börek suit different fillings. Other common ingredients that lend themselves well to gül böreği are beyaz peynir (white cheese or feta cheese filling) and minced meat (ground beef) and onion filling. Obviusly, you can combine the fillings. Potato and minced meat, spinach and cheese, etc. If you like Turkish aubergine recipes, you could use roasted aubergine as a filling. Or leeks. Feel free to experiment. But the important thing to remember is not to overfill the long rolls, otherwise the pastry will tear when you shape your rose.