The ethnic identity of the Huns is still open to debate, but Eastern Iranian isn't even in the running--they were either Turkic or Germanic. Sarmatians or Alans...would be weird but doable.
Yeah I'd lean toward Sarmatia as well. Although honestly, that whole civ is weird. It seems half-born of wanting to represent PIE culture (but having to fast-forward to recorded history), and half-born of merely wanting Tomyris in the game. My opinion is, there doesn't seem to be much outside of the Saka archer tying the Scythia civ specifically to Iranians nor the Massagatae specifically. "Scythia" in this instance is part-blob, part-region. "Saka" just means "Scythian," which is more-blob-than-region, but could still describe another horseback archer civ in the Scythian region like the Huns. Kurgans were also pretty wide spread around the Scythian region and used by Huns.
The ethnic origin is really the only major difference between the Huns and the Massagatae, and so given just how
generalized Scythia is I don't think the Huns would feel wrong as an extended representation of that region. If you look at Civ V's representation, not much changes. "Horse archer" becomes "Saka horse archer;" still mostly correct. Kurgans are still equally Hunnic. The Huns never had a city list and outside of Pokrovka and a couple far-east cities, the rest are geographically appropriate. The civ ability is equally Hunnic.
Furthermore, it would be one of the least offensive blob civs in Civ history. Present day Iran isn't in Scythia, so there's no loss of heritage in widening the ethnic representation. There aren't any surviving Hunnic cultures to my knowledge in the area, nor Xiongnu. In fact, the only thing that's mildly offensive about the prospect is semantics: calling the region "Scythia," if only because there isn't a better term for "That Steppe Nomad Region In South Caucasus Russia Between Kazakhstan And the Balkans." If anything adding the Atila (or a Sarmatian leader) would more clearly define "Scythia" and quiet complaints that Tomyris never lead the entirety of Scythia.
Babylon and Assyria as one civ, sure. Babylon, Assyria, and Sumer as one civ--one civ that is already horribly designed--would make me genuinely furious.
I'm afraid I have to agree with this, actually. Gilgamesh would make a great leader for a Babylon blob civ. But as a third choice, maybe a golden age leader, after the military and science leaders. Sumer is fine for what it's purpose was as an introductory civ, but that's the most compliment I can pay it.
Gotta love those Tupi street carnivals.
And their Minas Geraes. It's admittedly the weakest idea next to Kongo. Australia doesn't have any aboriginal representation in its uniques either. BUT I'd still play the hell out of a Brazil Tupi leader. And the effect of cunhadismo resulted in a substantial portion of modern day Brazilians possessing some Tupi blood; so they actually are still celebrating in the carnivals. It would be an elegant way of acknowledging the Tupi half of Brazil's culture, as well as another facet of colonial relations: integration.
But people are already complaining about Eurocentrism, so... 
Why not put Louis V in this way... 
Culturally, linguistically Assyria and Babylon properly belonged to the same civilization, and religiously it was just a question of who was top god: Marduk or Assur. I'd love to see separate civilizations, but I'd have no problem with seeing them as alternate leaders of one civilization if that's how we get both of them.
