You do realise that the tower fell destroying the whole fortress and all the forces in the battle were wiped out by Saurons destruction or buried in larva when the volcano went up, the only things on the plain that survived were the Hobbits that got air lifted out. The army at the gate wiped out the forces there.
Plenty of Orcs and Trolls survived in other areas but the MOS is dead meat.
There would be other Black Numenorians in the south that could fill the role your after, also there could be many lower powered evil Maia all around the planet perhaps in human or elven form, we know there were many Boldogs amoungst the orcs, one of them could rise in power and possibly take on a different form.