I've often wondered what kind of constitutional government would have made the Imperium Romanorum last longer than it actually did. It would have had to been introduced somewhat early on of course, but first, one would have to increased the authority of the Senate in conjunction with the Emperor. Maybe, even create several different senates in the major cities of different parts of the empire. The senates would have full domestic authority within their districts of control, such as public works, tax levying, and even command a sizable percentage of the Auxilia and a Legion each for defence, while the Emperor retains half of or three quarters of the Legions and a few elite Auxilia cohortes as a strategic reserve. The Emperor remains the overall head of state, military C-in-C and Chief Priest of the State Religion. But has no control whatsoever on the domestic and administrative policy of the individual Diocese of the empire, and needs the backing of the Diocese Senate before conducting a military campaign on their frontier. There perhaps should be an upper house of the Senate in Rome or some other better placed city, where representatives from the other senates meet to maintain cohesion and preserve the union between the seperate Diocese within the empire.
Any ideas?