In the early and mid 15th century, China had a huge navy, especially trading ships, and was rapidly expanding its international trade and influence.
By the beginning of the 16th century, religious influence had completely reversed that position, leading to the fleets being destroyed and foreign travel pretty much banned. China became extremely isolationist.
What if a different emperor had inherited the throne and resisted the clerics' policy of isolationism instead of embracing it? China would then have been on a par with the British empire in naval terms, probably military and merchant navies.
Or, going back a bit further, what if Emperor Marcus Aurelius hadn't broken with the adoption custom? That custom sustained the golden age of the Roman empire, through the time of the five good emperors. They were good because the first one, Nerva, deliberately scoured the empire for the man best suited to be the next emperor, adopted him and made him his heir. He then did the same in his turn, etc. Marcus Aurelius broke the custom and named his biological son heir. So the empire got Commodus, who was a violent raving nutjob who blighted the empire. Maybe the empire would have remained for longer if it had continued to have a reliable succession of good leaders.
Or the other way - what if Vespasian hadn't managed to seize the throne and stablise the empire? There were four emperors in one year, following years of rule by yet another violent nutjob. The empire was going down the toilet. It could have fallen apart completely.