Let's see, Ardashir started his uprising in 211 and became shah in 226. A missed opportunity for the Romans? Caracalla's murderer/successor Macrinus made war on the Parthians, the war didn't go too bad for the Romans, but for some reason he decided to make peace.
Caracalla wasn't really the most competent (except in killing and tyranny), but maybe? At least he could've conquered Mesopotamia for some time and imagine he was the new Alexander, as he liked to do.