Yeah, I realized that, thanks to the contributions of others

I'm still really excited about working with this, though. I think that perhaps a meritocratic tradition could be established by the hypothetical successor to Marcus Aurelius.
Okay: a little idea.
Commodus - whose caracter in this ATL corresponds with all prejudices and tales of the Historia Augusta and today's Holywood movies - incurs a premature death in some lower class/gladiator/charioteer brawl.
Hearing that, the emperor recognizes his son's psychic instability and inability to reign that he didn't want to see as long Commodus was his son and living heir. After a long period of grief, he decide to consider some problems the organization of the principate has.
Thus, Marcus Aurelius himself, one of these philosophers who are "incapable to reign and completly cloistered", develop a stoic political conception (the first complete model of political analyze Stocism produced). This new concept, known as "stoic meritocracy" or "stoic politeia", is exposed in the "Sermo de summa re publica", published in 179 AD.
It's basically a Roman form of confucian thought, composed and adapted without knowing its model. The founding principle is that each democracy, aristocracy and monarchy is doomed if the wrong leaders are chosen. In the book, Marc Aurel exposes how "the avaricious citizen of Athens destroyed their democraty, the narrow-minded oligarchs of Sparta led their city into a political desaster and the rotten oriental monarchy of the Persians was destroyed by the well organized, virtuous Macedonians."
The main point of the new ideology is the significance of good laws and decisions, established by appropriate, moral and complaisant leaders - wherever all the citizen are the leaders, or only some, or only one of them. Thus, each leader has to learn the described values - therefore the central meaning of education emphasized by Marc Aurel - and to demonstrate them in daily life and in a official trial.
The second part of his book - a more practical guidance - he describes his vision of the perfect Roman state. Because of the extent of the empire (and because the people was corrupted and unable to exerce power, as the emperor knew but don't explicitly wrote), Rome's government should be monarchic in the first line.
But the order of succession shouldn't be hereditary, practice perceived as dangerous and accidentally by the philosopher. Each emperor should, when attaining his office, propose a candidate for his succession, checked and formally elected by the senate.
As the emperor, all officials and army officers should, before beeing appointed to any position and office, pass an imperial examination established and controlled by the emperor; the examination should consist of wirting a philosophical essay examinated by aged examiners (the whole affair would in fact be very formalized; only one answer per question would be counted as correct, and the solution would have to be learned by heart).
The senate, consisting of the highest magistrates and official having passed the examination should be, besides its traditional and powerless formal sovereignty, an advisory body and a pool for potential emperors.
This model, despite being applied only slowly by the suspiciously imperial administration and senatorial aristocracy, is finally accepted as the Roman state ideology around 250 AD; postive effects of this reform are undeniably stability, a capable government and flourishing education; negaitve effects will be stagnation and a narrow-minded scientific world.