ok guys, please let me summarize this discussion, which gets a bit out of hands and offtopic.
The thread started with the OPs point about a better succession after Marc Aurels dead. Could this avoid usurpations as a main reason for the fall of the empire? Well, this just makes sense, if we believe that the 3rd century crisis was the beginning of the end.
In the further discussion we argued, that just another heir or a more stable succession model will not prevent usurpations. The reasons for usurpations are diverse. The political model of the principate is one reason, but also pressure due to internal issues (economical and social changes) and external threats (stronger Barbarians and Sassanids). And therefore, to avoid usurpations and the 3rd century crisis your alternate history has to start earlier than the era of the "Good Emperors by Accident".
Then the discussion went the other way around and a bit offtopic with the argument, that you could rescue the Roman empire later. That means, that the 3rd century crisis was not that detrimental and the empire of the 4th century was again strong, prospering and defensible. If you agree with the shock-theorists you just have to avoid some single events, in order to let the core of the WRE survive, which is at least Italy and Africa.
The point, I tried to make is, that it is not sufficient, that the WRE survives with the chance to grow again slowly in order to rescue the ancient world and avoid the so called dark ages. It is also neccessary, that the ERE survives as a real empire controling the orient. As long as the Mare Nostrum survives as the cultural hub of the ancient world, the ancient world survives. Or at least we can expect a smoother transition into a different mid-age. BTW, another option to rescue the ancient world is, that the muslims are more succesful and conquer Europe. But thats another story.
Now I played the advocatus diaboli in this threat, defending the pessimistic view. If we assume, that the WRE with Italy and Africa still exists in 476, it still meets some serious challenges:
- even if the Visigoths and Franks probably might stay where they are, there are a lot of other german tribes which might invade Italy. Think about the East-Goths in Pannonia and the Langobards and others north of the Alps. Also new steppe tribes will arrive like the Avars. And Africa is also under permanent attack by the Berbers and others.
- the WRE has to solve the problem with their overwhelming barbarian magister militum. One option is, to disempower and control them, like the ERE did. But due to the different social and political structure and situation I doubt this is possible.
- The second option is a german emperor. Well, we had something similar in OTL. Odoacer took over the reign over the rest of the WRE in 476. He did not ask for the title Augustus or Caesar, just for patricius. For the ERE this allowed the illusion, that Italy is still a province of the Roman Empire governed by a foederatus. Which was nothing new. Same with Theoderic the Great. If there was ever a german emperor of the WRE, it was he. Looking to his measures, he was better and more roman than the most emperors of the last century.
But again, he was not called Caesar. The ERE saw no need to appoint a western emperor, in order to just rule one province. And the East got just rid of their usurping german magistri. They were definately not interested in a german emperor in the West, also in order to prevent new ambitions in the East. I expect a war, if a german appoints himself emperor of the WRE.
- another problem is, that the WRE in the early 5th century ceased to exist as a state more or less. The emperor lost all the important social groups. The landlords did their own thing, trying to avoid taxes by all means. The same counts for the warlords. The soldiers were mostly foreigners loyal to their warlords. Even roman soldiers were loyal to their roman warlord and landlord and not to the empire or the emperor. The rural plebs and cities looked for safety and care by becoming clients of the landlords. The urban plebs in Rome lost any connection to the emperor in Ravenna. Therfore the emperor of the WRE has to rebuild something like a state from scratch.
So my point was, that the WRE will perhaps be too busy in stabilizing and defending itself in order to be any help for the East. That does not mean, that I don't believe, that the ERE could survive and beat the Arabs. Perhaps we could avoid the war with the Sassanids. Perhaps another miracle was close to happen by just changing one single event. Please, don't say Mohammed dies as a kid, thats too easy. Like the idea, that the WRE survives and everything becomes better is too easy imho.
Actually I like the idea of a surviving WRE in the 5th century and a surviving ERE in the 7th century. The world would be very different and a bit more ancient in the next 1000 years. However, I like the idea to change the roman empire earlier (1st cenury BC or AD) much more. Coming back to the OP, I guess the discussions show, that Marc Aurel and his successor are not that important. and the 2nd century is not the best point of time to change roman history.