Challenge: What change, if any, in the drafting of the Constitution *which could plausibly have been adopted and ratified in the late 1780's* could have averted the ACW? It is hard to think of any change directly related to slavery that would not have fatally alienated some southerners or northerners whose support was needed for ratification (e.g., an explicit prohibition of, or guarantee of, slavery in all federal territories, or a change in the way slaves were counted for purposes of legislative apportionment) so it probably has to be something with a more indirect effect. For example, the small states, worried about the power of large states choosing their electors on statewide tickets, insist on a requirement that electors be chosen by congresional district, which would have made it harder for the Republicans to win the presidency in 1860 or maybe even thereafter.
Of course such a change might have plenty of earlier consequences, so you may say the Republican Party as we know it might not have existed in 1860. The basic point is the same, though--it would be harder for any anti-slavery party to win the presidency if it lost a substantial number of electoral votes in the North.