You actually reminded me, given the scope of devastation the south is probably going to see some demographic changes (considering there wasn't really all that much otl surprisingly). Blacks made up the heavy majority of the (lower) Mississippi valley and some parts of the delta (factoring Creoles and Cajuns), the Alabama/Mississippi black belt, and the coastal strip of Virginia, N.C., S.C. and Georgia, and southeast Texas (basically all the areas in the south that have both fertile and flat land). The greater devastation ttl will kill lots of southern whites and probably displace much of the rest of their population, so by the end of the war I see whites being basically eradicated from all the slave-majority areas I listed earlier. This gives the federal government after the war a chance to permanently reshape the south's demographics to ensure they never rebel again.
Forty acres and a mule was a noble gesture, but most of the north opposed it (they didn't want to have blacks settling the midwest with them, sadly), and if it had been attempted would have led to starvation as the resettled slaves would be essentially dumped there without the knowledge of how to grow most crops. However, at this point there is still plenty of arable untilled land in the lowland south (especially in the Mississippi Valley), so the federal government as part of atl "reconstruction" can just relocate the remaining southern whites to the highland territories (Virginian and Carolinian piedmont, central Tennessee, northern Alabama) and declare all the black majority territories I mentioned above as free land to the now-freed slaves. This solves most potential white v. black violence but more importantly, it both irreversibly shifts the plantation economy to be non-existent AND settles the question: "what do all these people do now that they are not slaves anymore". The north and midwest will be happy as they didn't want any black migrants, and with the white population so devastated they can ensure that southern "democrats" will never dominate congress again.
A detailed proposal would be: "Confederate sympathizers" would be "evacuated" from the former slave areas and moved to the "upland" areas of their states to ensure their "safety" from the freedmen. In return, the freedmen would have permission to develop the land they live on and live as they please. Western Tennessee, southeastern Missouri, northern Louisiana, and the
Arkansas Delta are ceded to the (now black) state of Mississippi, western Virginia is broken off as otl, and the Appalachian parts of North Carolina, South Carolina, Georgia, and Tennessee are broken off and combined as a state of Appalachia as a reward for their loyalty with the capital at Knoxville (these areas were heavily unionist). Now North Carolina, South Carolina, Georgia, Alabama, and *expanded Mississippi are permanently majority African (IE: unionist). Virginia, Louisiana, Arkansas, and Florida are not majority black but are still heavily affected by this as to mostly destroy the "southern" way of life.
The advantages of this plan are that no old state is completely broken up (a major legal concern) and that nowhere in the north (in theory) has to deal with any refugees/freedmen, and the idea of the "south" is more or less destroyed. So in the Fed's eyes, it's a win-win scenario. I'm pretty sure there are flaws with this somewhere, I would like feedback on this idea.