Commodus had a twin brother named Fulvus (means blonde, so presumably they were fraternal as a cognomen like that wouldn't make sense if they were both blonde) who died when he was 4. A perfect POD would be for him to live and Commodus to die. Given his youth when he passed, he could have matured in any fashion an author wished. If he was a chip off the old block instead of a wackjob, Rome would be in much better shape in the 3rd century. He would be 19 when he came to power in 180AD and if he died at age 70 he would reign 51 years.
I assume he'd marry the same woman Commodus did, it was an arranged marriage. They never had any issue and Commodus is said to have preferred men, but I'd be surprised if he never performed his...husbandly duties, the Romans had a flexible view of sexuality. So, if Bruttia Crispina remains childless or only has girls then Fulvus would be able to adopt a successor of his choosing rather than rolling the genetic dice.
Marcus Aurelius Antoninus Augustus, 16th Emperor of the Romans lay dying. Yet he had not a heavy heart. His son was by his side, his tawny blonde hair hanging in front of reddened tear filled eyes. He was a young man, of just nineteen years, and he was not the philosopher that Aurelius would have wished for. Of course few young men were. He was a good man though, fair and just with a charisma that made men twice his age and more want to follow him. And lead he could, his talent was clear. At politics and administration he was skilled, but at tactics and strategy he was gifted. Aurelius had been a competent general, but he had no illusions that he was especially skilled at the art of war, his son had already outstripped him in that field.
That had been clear since that day last year when he’d saved the right flank of the army, and turned incipient defeat into a crushing victory over the Marcomanni and their allies the Quadi and Iazyges. The legionnaires had made him a grass crown as done for the Republican heroes of old, and much was spoken of the fact that he was the same age as Alexander at Chaeronea when he’d done the same with his father’s left.
Marcus Aurelius could feel himself slipping away, if anything needed to be said it had to be now. He reached out a trembling hand to his son. “Don’t let it slip away…” he rasped. “You have to finish them…preserve what we’ve gained…safeguard Rome” He was coughing wetly now.
His son grasped his arm with the strength of youth. “I will father,” he said intensely. “Sarmatia*, Quadia, and Macrommania, they will all be made provinces as will all of Germania Magna west of the Albis. And if the Sassanids bother us again I will march east and end them once and for all. What Varus lost and Hadrian gave away I will return to Rome!”
The Emperor’s arm went slack and his eyes glassed over, but Fulvus was comforted by that slight smile that had graced his father’s face before the end. He gently lay his father’s arm down and closed his eyes. Standing he rubbed his tears away and composed himself, he was the Emperor now, and he had his duty. He strode from the room. “Prepare the Legions, we march.”
* Land of the Izagyes, south of the Quadi and between Dacia and the bend in the Danube. There were other more powerful Sarmatian tribes to the East, but that's what the Romans planned to call the province.