I always thought that if there had been a second Trajan, a lot of the problems of the 2nd century would not have happened.
Trajan was one of the best show-runners Rome ever produced, and was getting set up for the final push to bring Parthia into the empire as a set of provinces. Then he up and died. I’ve viewed that with as much suspicion as the earlier sudden death of the young Alexander the Great.
One of the really inspiring “what-if” scenarios is, Trajan lives, finishes off that last serious long-term enemy of Rome and opens up the direct overland trade routes with China and India (they had sea trade, and with India and points as far east as Sumutra that was quite substantial). He had spent a lot of time and gone to some trouble to conquer Dacia, and clearly didn’t see or wasn’t intimidated by the arbitrary limits some set for expansion.
Of course, we don’t get to know the road not taken, too bad about that. :’(