I think that almost anything can go in a burrito. You need a reasonable mix of carbohydrates, protein and fresh vegetables. Stuff shouldn't be too dry so that it falls apart, or too wet so that it drips everywhere. And the mix shouldn't have overly big bits which prevent you getting an easy mouthful.
Rice is good because it is cheap and holds things together.
Scrambled tofu has a good texture and provides protein.
Fried potatoes give interesting texture and can add flavour if fried with spices.
Sliced avocado is big, but can be easily bitten off. Chunky guacamole also works.
Beans can be mixed in with other stuff, or as a seam running through the burrito.
Lettuce can provide some crunch and make the burrito lighter.
Baby spinach wilts a bit with the hot ingredients and provides a good texture.
I had quite a few burritos in California and Nevada. Lots of the places offered a choice of beans - black, pinto or refried. The salsas are very fine and hotter than ours. Eric tried liquidising our salsa, but I think there is too much tomato and juice, so it becomes very runny. I think that if the salsa is fine, it should be hot so that you don't need so much.
American burritos often come wrapped in foil, or come with nothing on the outside. I didn't think I'd like this, but I did - it makes it 'street' food. They are made with hot ingredients and rolled tight, rather than reheated, which makes them fragile and loose, requiring a plate. Cheese, if used, is put on the wrap first and melted a bit.
Using hot ingredients means that they can't be rolled in advance. But the filling can be microwaved and then rolled up, so it shouldn't be longer to make than just now.