From the perspective of someone with less-than-stellar grades, who just made it through admissions this season, and who isn't from an ivy league school: You need to cast your net a bit broader than some, but you're in a pretty good place. It might be worth it to complete an MA first and hone in on your writing and pick a topical subject. This process of generating a writing sample goes best if you are working with a faculty advisor, and spending a good deal of time on it. Your writing sample, along with stellar MA grades can swing the outcome of applications drastically.