As an aside, a Roman citizen may not have been born in Rome.
As well as that, they may have had no Roman or Italian ethnicity.
A couple of famous examples:
Emperor Lucius Septimus Severus who was born in Leptis Magna which is in present-day Libya. He ruled the empire from Britain. He was the subject of an audio drama "Severus", co-written by Paterson Joseph, who also played the title role, broadcast this afternoon on Radio 4.
https;//www.thecollector.com/septimus-severus/
Saint Paul the Apostle. Born Saul in Tarsus, which is in Turkey.
My Irish grandparents were British citizens (or subjects of H.M. the Queen/King) because Ireland was part of the British Empire when they were born. Some of my older Irish-born cousins also qualify as British citizens. Younger Irish-born cousins qualify through a parent. Some more cousins are British citizens because they live or were born in Northern Ireland. All my other (English) cousins on that side of the family are British citizens by birth but are also eligible to apply for Irish citizenship.