If a smart Emperor could face some sort of legal requirement to pick a smart successor, than how is this going to screw up?
maybe...
In a better, and pretty unroman world, the requirements would be:
1. a guy of at least consular rank. With the officially required old minimum age of 42. Not a youngster like Gaius, Lucius or Germanicus pushed thru the cursus honorum, without any serious assessment.
2. a guy who is backed by the majority of the senate
3. a guy who is beloved by the plebs urbana
4. a guy who got a lot of experience in civil adminsitration
5. and military camapaigns: the legions love him. The generals respect or fear him.
6. a guy who respects the emperors daughter and marries her
7. a guy who is wiling to adopt the emperors son and favor him over his own son, if it comes to further succession
Honestly. If such a guy exists, he should had killed the emperor already
Just an example: When the senate finally appointed Claudius emperor. Who should enforce Claudius, backed by the praetorians to adopt a man of consular rank like Iunius Silanus, one of the last descendants of Augustus? Right here and now in January 41! Instead of tinkering with kids like Britannicus and later Nero? Who got the power to enforce Claudius? Just in order to avoid an incompetent and overwhelmed kid on the throne.
However. If you like to reduce civil wars. You must enforce every emperor to appoint an experienced guy directly at the day of the emperors inthronization. Without an already appointed adult successor, you risk, that senatorial legates with legions start a civil war, if the emperor dies without an already accepted heir.
But does this really reduce civil wars? Or would'nt we get even more civil wars caused by ambitious co-emperors? According to the roman mindset, the first idea a true roman, appointed co-emperor should have is, to kill the emepror asap.
Remember, the so called "Good Emperors of the 2nd Century" have just been lucky, infertile and sometimes incompetent little bastards. A guy like Lucius Verus was a great expception and a very unroman bon vivant. Perhaps the reason why Antoninus and Aurelius did not kill him. He was the perfect co-emperor I described above: Loyal and fully harmless. But imagine Marcus had died by the plague and not Lucius. And now Lucius must do the job. What a desaster.