 Fundamentally you can use anything you like - it's your food! However, in Japan if you make a ball of rice (which is often sweetened itself, and pounded partly or fully to a sticky paste or dough) and filled with something sweet it becomes confectionery or __wagashi__. Some wagashi that take the rice-with-sweet-filling form include daifuku, mochi of various kinds, _yatsuhashi_ and so on. The wagashi that's closest to the idea of a sweet onigiri is probably _ohagi_ or _botamochi_, glutinous rice "onigiri" of sorts which are filled, covered or both with sweet bean paste, kinako (toasted soy bean powder), sesame seeds and so on. So, to Japanese sensibilities an onigiri is something savory, not sweet.  Fundamentally you can use anything you like - it's your food! However, in Japan if you make a ball of rice (which is often sweetened itself, and pounded partly or fully to a sticky paste or dough) and filled with something sweet it becomes confectionery or __wagashi__. Some wagashi that take the rice-with-sweet-filling form include daifuku, mochi of various kinds, _yatsuhashi_ and so on. The wagashi that's closest to the idea of a sweet onigiri is probably _ohagi_ or _botamochi_, glutinous rice "onigiri" of sorts which are filled, covered or both with sweet bean paste, kinako (toasted soy bean powder), sesame seeds and so on. So, to Japanese sensibilities an onigiri is something savory, not sweet. 