One way to make a burrito lower fat is to use a low-fat
tortilla. Standard burrito-sized tortillas are up to
350 calories of largely fat. The lower-fat types suffer
somewhat in texture, but they taste fine.

As for the fillings, "bean burrito" for me conjures up an image
of a burrito containining pinto beans the are at least
smashed if not refried, possibly contains cheese, probably
seasoned with dried chili powder and garlic, with minimal
other ingredients, and possibly the whole thing fried
("chimachanga" style).

If you stray too far from this and add other stuff you will have a
vegetable burrito, or a meat burrito instead. However you could
include finely chopped and sauteed onion, peppers (tinned serranos
are good if you like heat), and zucchini in the right proportions
and not subtract from the bean-ness of it.

Steve