There were always foreign troops in the Roman army. They were in units called Auxiliaries. Roman citizenship was granted to an Auxiliary who retired in good standing.
I know, but regular legionaries were Roman citizens, which the Italians early became.
If I recall correctly, the original question was whether the Roman Empire could have survived if it had transformed into a pluralistic democracy.
Roman citizenship was valuable because it exempted a person from local laws. Any complaints or charges against a Roman citizen had to be brought before a Roman Magistrate. If everyone in the Empire was a citizen, the relative advantage of being a citizen is greatly reduced.
Beyond that, Rome's wealth was greatly derived from heavy taxation of the provinces. If the provinces had effective political power, the first thing they will do is cut off the flow of money back to Rome. In the later years of the Empire, this actually became a factor in tax collection. Many legionnaires were granted land in France and Spain as their pension. Roman tax collectors found it much more difficult to extract money from Roman veterans than they had from French and Spanish farmers.
If we take away the special privileges of being a Roman citizen and greatly reduce the theft of goods from the provinces, the Empire cannot hold together.