Alright, surely there are a lot of Roman experts in this forum, so...
To keep up with this challenge, did a change in Roman constitution is necessary...?
AFAIK, things like rebellions, civil wars, and usurpations were common throughout human's history...
So IMO without a serious external threats like Germanic migration and Arab invasion, Rome could more focusing her attention for internal threats...
This way, present day Roman Rhine, Danube, and Euphrates is not unlikely, thus fulfilling the challenge...
Am I wrong...?