I myself, am not too interested in the black missions, I am more of a green mission lover, and the occasional red mission if the team is there.
But... If I were to be doing black missions, The difficulty of the mission might help what the crate spawns.
Green mountain I believe is the easiest black mission and I don't think it the loot should be too op

Possibly some building materials and some more sought-after ammo types. So maybe some concrete walls, Full metal floors, Safes, M2 Machinegun ammo, M134 ammo, Possibly make this one an IKEA crash with ammo?!
Devils castle is a bit harder, So the loot may correspond to that difficulty? Devils castle is where I saw the real power of the M2 Humvee, So going along with your previous post sand, Maybe a lockable M2 humvee spawn along with the loot? Base building materials in a larger quantity, and again, Ammo?
Now, Skalisty island..... Mother of god.
I myself have seen the true potential Skalisty has to F*** up even the best and most cautious of players.
To go back on the spawned in vehicles, Something like an ASUV and a Radar chopper would be cool to have spawn with the Skalisty mission, The crate would be a different story.
For the crate, I would love to see just a massive amount of building materials, Ammo for the vehicles, And possibly something special, Like a launcher? I know I would personally love to see a javelin in that loot. (I think those are in the game?) If not, I would love to see a stinger, Something that really sets aside the boys and girls from the women and men.
There ya have it, My opinion on the loot spawns for the black missions!