Once a slob, always a slob. Never knew someone to suddenly become neater because a room mate (or for that matter, spouse) didn't like it. When you choose that person's replacement, make it clear, in writing if necessary, what exactly is expected.
My husband of 30 years and I joke about our "pre-nup". One rule was that s/he who cooks doesn't have to clean. Yes, at this point (over 30+ years, we lived together for a couple of years prior) that means I cook and seldom do dishes. I do get stuck with some of the more tedious pain in the butt housecleaning chores, but when I ask him to do one of them, or we do what we call a "white tornado" in advance of house-guests, he pulls his share.
By the way, another of the prenups was bill paying. I'd write the checks, he has to balance the checkbooks.
Another was "s/he who is doing the job is doing it correctly, period." That is to say, if you don't want to do the laundry, don't complain about how your Tshirts are folded, etc.
None of these were written, but they were discussed and agreed to before we moved in together, and put into practice before we married. This is what you need to do when you get married. But a room-mates agreement? Watch Big Bang Theory (I don't know how to insert it, but picture me laughing uproarosly.