There was a Roman emperor, Gordian III, who either died in battle (easily butterflied) or was assassinated (harder but still rather easily to butterfly away) after having taken the throne at age 13 in in 238; he ruled 6 years and it seems like much of that was spent at war.
My question is, what if he lives? I was tempted to give him 60 years, but that would make him 73 and maybe getting a bit up there for the stress he'd be under if there was a lot of war. 50, however, puts him at age 63 which is an age when several emperors were just starting, and while old not ridiculously so for an emperor of those days.
This gives him lots of time and also allows Rome, which suffered a slump in the 3rd century, to avoid a period of a lot of assassinated emperors and instead allows him to gain favor with a lot of groups. Does he divide Rome a century early? Does he stabilize borders? Even try to establish buffer states? Would he continue to tolerate Cbhristianity as was done more or less during this period (though one could argue becasue rival groups were too busy persecuting each other)? Could he even convert, strange as that would seem nearly a century early?
This could make for an interesting timeline to see an end to the troubles of the middle 3rd century. Even if the TL has to go a few years further back than just his not dying. (After all, he's a teenager and therefore could easily make different decisions.)