I think the Romans could have easily conquered the Parthians if they used NATO Combined Arms tactics.
Skill issue.
Honestly, just a political system that’s more stable would have gotten them most of the way.
“I’m sorry, it sounds like you expressed dissatisfaction with the ruling government. Wouldn’t that make you… EMPEROR?” - Your Rhine Legions
It sounds like even a little more mobility into the senate would have made a difference. Would the future barracks emperors have revolted if they were allowed a say in the next emperor, anyway? We’ll never know.
Or maybe you’re talking about Saturninus. Was that an actual significant challenge to the Empire? It sounds like it was put down pretty easily.