It's probably been done a few times before, but a supposing worth refreshing, given the bitter-sweet contemplation of potentially a better American society than what transpired.
A key stipulation, both on my personal part, and what I believe realistically what would be most likely a consequence, is the quick demise of slavery - no backing in of slave states into the Union.
So, decisive victory by the Union at the 1st Battle of Bull Run? Or the real destruction of the Army of Northern Virginia at Antietam? Or perhaps the First Battle of Kernstown going far more badly with Jackson dead?
Since there has been a great deal of creative energy at play in working out various speculative Confederate victory scenarios, it would be interesting to deal the deck from the other side.