Just like we had 3 civs in Dynasties of India, we could have had 3 civs here: Armenians, Georgians and Wallachians/Romanians (same thing). It’s still the same geographic area, around the black sea, and the carpathian mountains still fit with the Mountain Royals name.
It didn’t even need a campaign, it already had Vlad the Impaler’s campaign. 
The Romanians (or Vlachs, as I call them) would fit better in a Balkan DLC. The Caucasus is quite a bit different.
I don’t think we’ll ever have a Serb-Croat DLC since we already have the Slavs civ, and even if they do, maybe they make a Slavic DLC excluding Romania there too. So likely everyone in the region will be implemented in the game before Romania is.
I wouldn’t be so sure. The Slavs mainly only represent the Ruthenians, and a lot of people want them renamed. So that would leave room for a Balkan DLC…once they fill out other architecture sets first.
Given that they went back to 2 civs now, they’ll likely make a Croat civ and Serbian civ leaving the Wallachians on the outside again.
We need the Khazarian Empire! not some tiny failed rebellion. We need armored Rabbis riding on the open Steppe more than a vampire with sticks.
Tibetans would work better with the mountain theme I feel, I would like to see Balkan Civs at some point though
East European has only 6 civs. It can easily accommodate 2 more to bring it on par with Mediterranean
Vikings still lack a campaign so maybe the dlc that fills that will include romanians to correct the dracula campaign.
probably because romanians dont live in mountains
Because very few people are asking for them and they’re unrelated to the Caucasian theme of The Mountain Royals.
The theme of the dlc is the Caucasus, not the Black Sea which the Armenians didn’t even lived on. The Romanians wouldn’t fit.
That’s just doomsaying, we also had the Slavs when the Bulgarians, Poles and Bohemians were added so I wouldn’t be surprised if the next European dlc is themed after the Balkans and receive the Vlachs, Serbians and Croatians as new civs (or maybe the Serbians and Croatians would be grouped together as the Sclavenians and we would also get the Albanians) and the Magyars would receive a new campaign. That makes more sense for a DoI style expansion.
Doomsaying again, as you said adding the Romanian/Vlach civ would save a slot for an additional campaign for the Magyars or Slavs (or maybe even the Turks, but I’d rather bet on the Magyars) so I see them as being quite likely added in such dlc.
Swiss Tibetans also live on the mountains.
Swiss formed a republic, they never had royals xD
More seriously, the theme is the Caucasus mountains, not every single mountainous people. The Wari, Nepali and Kashmiri also come from mountains and nobody expected them to be cramed in with a Georgian and Armenian dlc.
https://www.researchgate.net/publication/2517680_The_Forgotten_Black_Sea_Armenians
There have been significant Armenian populations in the Black Sea. But yes, their culture was mainly in the Caucasus, the Armenian highland and to a lesser degree Cilicia.
Why are you never satisfied?
It would be nice to have Tibetans civ, but it is unrealistic - as long as AoE wants to be on the Chinese market.
Spiliting of Vikings umbrella civ is the most obvious opportunity to add a Viking civ campaign.
A common civ for Croatians and Serbs would be a failure - despite the common language, they are completely different.
Albanians could be represented by the Venetians civ - using the Stradiot unique unit.
In fact, the title of this DLC would fit both the Caucasus and the Himalayas or Andes - Forgotten Mountain Empires DLC 
Why would including the medieval Tibetans as a civ be a problem in China? They appear in Chinese history books.
Or what about another Middle Eastern civ being added to AoE2 like the Kurds?
At the rate world events are happening I doubt they will get added.