What if the Constitution specified that slave states could not be admitted north of the Mason-Dixon Line and no attempts to prohibit slavery could be made by the Federal Government?
Then the Constitution doesn't get ratified. States south of the line would be unhappy, as well as states north of the line: Pennsylvania, New Jersey, Delaware, New York all had legal slavery prior to 1800.
Also, that's a plain contradiction. "No attempts to prohibit slavery" + "Constitution prohibits slavery north of a certain line".