Question: Vegan Onigiri Fillings

Greetings, I am a vegetarian that has started experimenting with making ongiris. I am having fun, made a good amount of Simple rice ones with some sesame seeds, but I was wondering which could be some good inexpensive VEGAN options for fillings?

also, if the onigiris feel a bit dry, does that mean you lacked water? how do you know if you put too little water? Just made a batch today that felt off.