Another set of suggestions.
As the Scourge are no longer affiliated with the Burning Legion they have no Dreadlords anymore. As a replacement they should get the San'Layn since they are rather similar.
http://www.wowpedia.org/Darkfallen
http://www.wowpedia.org/San'layn
But anyways, if both the Scourge and Forsaken are going to be in this effort must be taken to differentiate them as much as possible.
The Scourge should be the monstrous near mindless horde of mutated zombies and undead monsters it has always been, with necromancers to raise weak undead, while the Forsaken should be more a disciplined faction of non-mutated undead with advanced chemical weapons, with less focus on raising the dead.
So where Scourge have Ghouls, Forsaken have Deathguards. Where Scourge have Necromancers, Forsaken should get Shadow Priests or Apothecaries(Also an idea for a hero unit).
Since Forsaken have no Nerubians they would simply have some sort of archer, who perhaps poisons their arrows as an upgrade. Not Dark Rangers however, as they are not common enough.
And as I suggested earlier, instead of Meat Wagons they get Blight Spreaders.
http://www.wowpedia.org/Blight_spreader
As for a flying unit to have instead of Gargoyles, I really don't know. Someone should really start trying to figure that one out. Perhaps some sort of Bat rider who throws Blight bombs at people.
Also as I also said earlier, these should be included somewhere
http://www.wowpedia.org/Forsaken_Plaguebringer , as well as these for the Scourge
http://www.wowpedia.org/Plague_eruptor
Anyways these are just ideas. But still, it would be wrong to make the Scourge and Forsaken too similar. The Scourge should be more number focused and the Forsaken more about strategy and intelligence.
In style they are two very different types of undead, and that should be kept in mind.
Even if both were to get Abominations and Banshees effort should be made to make them somehow different, even if only in some small, subtle way.