I had an idea for a timeline a few months ago where there were more free states during the Constitutional Convention than OTL. It was a bit of an Ameriwank, where Canada, St. John's Island, and Nova Scotia ended up with the new US by the time the Revolution ended. Also, borrowing an idea from another persons timeline, Georgia remained a free state due to an earlier gold rush in the 1740's.
So there were three big provisions dealing with slavery that I wanted in the new Constitution. One is an earlier end to the slave trade, either set for the date of ratification, or maybe a little later like between 1790-1795, but earlier than 1808 in OTL. The second is the banning of slavery in any new state added to the union. I think these two could be done with a greater free/slave state ratio.
The problem comes with the third provision. I wanted it stated in the Constitution that any person born after the date of ratification would be a free person. I thought it would be a good way to phase out slavery in the remaining slave states, but after thinking about it for a while something really sinister came up. Just imagine you were a slave holder in one of the few remaining slave states, and after the Constitution is ratified you realize that any child born from one of your slaves would be free. How would you deal with the possible added cost of raising a child, while losing out on revenue of their future labor? This lead me to thinking that we'd possibly see mass-sterilization of slaves in those states. Is that a realistic possibility as a reaction to my anti-slavery policies, or am I just being too pessimistic in regards to human behavior, and that it would never get that bad?