What a question...
I have another rhetorical question:
What is a Romanian?
The only answer that I could think of is: a human with Romanian as mother language.
Can we go further to ask ourselves if both parents of this guy should be Romanian native speakers?
We may agree that is hard to define ethnicity based on ancestry.
Romanian linguists cannot agree when the Romanian language appeared in Balkan Peninsula, as a Romance language derived from Vulgar Latin.
Some say 7th century, others 8th and others 9th century. Let's take 7th century as reference.
Linguists use the conventional name 'Common Romanian" or 'Proto-Romanian' to identify that language.
Romanians and Aromanians of today are the descendants of the latinophone population that inhabited the Balkan Peninsula between 7th - 10th century, living North of
Jireček Line - Wikipedia.
If we go back in time, that latinophone population had as ancestors Thracian and Dacian people, but also Roman colonists which came from the entire Roman Empire.
Can you imagine how this mixed population looked like?
Aromanians are first mentioned by the Byzantine hystorian
George Kedrenos - Wikipedia in 976 AD as a latinophone population near Lake Prespa and Kastoria (thus near today tripoint border of Albania, Macedonia and Greece).
From this 10th century until today the Romanians, Aromanians, Megleno-Romanians and Istro-Romanians lived in separated areas, while the rest of the Vlach population was assimilated by the Slavs.
Without having historical sources we may guess that during Middle Ages the Aromanians (or Vlachs) could have assimilated some Greeks or Slavs
and (with higher probability) a big number of Aromanians were assimilated by the Greeks and Slavs.
But a theory like the one developed in Greece during 19th century, that (all of or most of) Aromanians are descendants of Greeks that were assimilated by Vlachs many centuries ago, while the Vlachs who assimilated them have been totally assimilated by others (probably by Slavs) is based on nothing:
- I don't think there was a single Middle Age historian interested in researching the assimilation of Balkan people during his lifetime, so that he could record in some documents such phenomenon (while such assimilation process must have happened during many generations)