Let's assume a postulate from this answer happens:

small explosions cause small leaks [in the Manasphere]. So cannon fire causes Mana to leak in for a while. - Darcy Thomas

Let's assume this is true. That of course leads to a sudden influx of Mana in the world around the year 904 AD / 3600 Huángdì era, when Gunpowder was used militarily for the first time. The one controlling the gunpowder would most likely be either the Tang Dynasty (which ended 907 AD) or one of their successors, and they will use it either to keep china united or to overthrow the Tang and become the new Emperor of China.

The magic-system will develop uses deliberate explosions and channeling the inflowing mana through the hands of Taoist priests. At first, the results are rather messy, as unconstrained mana detonates, turning compressed flasks of gunpowder into the equivalent of more modern hand grenades. But once they get a hang on channeling it, the resulting magic will be in line with typical Chinese high fantasy magic, such as throwing lightning from their fingers, punching a whole group of people by hitting the air or, empowering their body to do near-impossible feats.

As a side effect, the gunpowder requirements for firearms would be reduced, allowing to supply more shots but also making handling the powder much more dangerous.

Let's consider the (re)unification shall be done by about 954 AD / 3650 Huángdì era, 50 years after the first use of gunpowder in the military, under a Chinese splinter state/dynasty. Because the Mandate of Heaven is slow to change, the general political system of Emperor and surrounding tributaries that has worked since ages won't likely change too much - for the foreseeable future. On the other hand, the social structure would adapt to incorporate the gunpowder-taoist-battle-monk/magicians by simply altering their position some - assume that their power grab to but some Taoist Superpriest onto the throne will not happen during the unification period.

Would the magical Tang Army manage to keep up the peace or does the empire still fracture and one of the Qi/Jin/Yan/Wu/Chu/... armies does manage to overthrow the others, as their agenda might align better with the Taoist philosophy? Will one of them have a Xianxia/Xuanhuan-army with a monopoly on gunpowder ow would the history of this period just happen as it had in our world?

Notes about contemporary Events in Europe

We are talking about the timeframe of 904 to 954 here. We are pretty much in the high of the Viking Era. In Britain, the biggest event in that timeframe is the grand heathen army establishing the Danelaw (destroyed in 954). France ceeds the Normandy to Vikings aka "Normans". The Bretagne is still independent, the HRE is it's main counterbalance and spans from Hamburg to Rome. Burgundy is an independent nation, Spain is mostly Muslim. Russia is in no way united, the Byzanztine empire is still in existence.

Why the cutoff? Some events that shape our world a lot would happen a lot differently now, so cutting the timeframe short makes it easier. Two large events much after our first cutoff for eastern Asia that would ripple well to us: Our World's Djingis Khan unified the Mongols around 1200, but with a magical China's Xianxia army, the Mongol invasion of China would happen a lot differently - depending on how China turns out in this question. Majapahit's rise to power around 1294 was influenced a lot by the then ruling Mongol Emperor in China, and his soldiers leaving behind the secret of gunpowder - and thus putting Majapahit into a powerhouse situation in modern Indonesia.

  • $\begingroup$ In relation to @L.Dutch-ReinstateMonica's comment, take a moment to read How to deal with "Please Develop my High Concept" questions?. A High Concept question is one where a small change leads to very broad consequences and the question wants to (proverbially) know them all. Rewriting history fits into that category because it's not objectively specific (and very hard to be). If you think about it, it's basically asking respondents to write an entire book. $\endgroup$ – JBH Oct 12 '20 at 17:00
  • $\begingroup$ so the gunpowder still act like gunpowder right? can civilian use it for making fireworks/firecrackers or its full on military only? also do you mean other kingdoms also get their own magician or magic due to the mana spreading to entire world or only taoist can? also do the magician need to take time in activating the magic? like mumbling the spell or make certain move or its instantaneously? can the magician stop or protect themself with magic against bullet or rocket arrow or the gunpowder explosion it self? $\endgroup$ – Li Jun Oct 12 '20 at 18:02
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    $\begingroup$ You have just created the world of "Avatar the Last Airbender" where China is similar to the Fire Nation. The Mongols would not have a chance, and would probably be conquered by China before even creating their empire. Central Asia would probably be more prosperous due to not being destroyed. Probably Manchuria and Korea would be taken by Chinese as well. I don't know about Tibet or southeast Asia. This magic probably would fuel industrial developments, as in the story of The Fire Nation, leading to greater overall military strength. In the long run world becomes Easternized instead. $\endgroup$ – Galaxy Oct 12 '20 at 22:01
  • $\begingroup$ @Galaxy i delete my comment since i forgot that OP specifically talking about tang dynasty rather than the era where genghis khan finish unified mongol, since if we talking about that dynasty the china is fractured into 3 dynasty the mongol still has chance. regarding tang many neighbour kingdom or empire is pretty much idolize china, some even think themself as the true han due to china is conquered by barbarian larp as han, until european conquer during Qing decadence and the people is addicted by drug and dumb policies which i doubt such history will change in that situation. $\endgroup$ – Li Jun Oct 13 '20 at 8:27
  • $\begingroup$ @Galaxy even if tang was still around during that time sooner or later decadence happen, besides tang did vassalize korea and conquer other proto mongol due to their emphasize in cavalry unit, the rest probably just keep giving tribute like usual, cant say they can conquer tibet though due to the terrain and the type of unit they use, and tang problem is due to the imbalance or overpower military governor turn against tang and rebel which fracture china into multiple dynasty and kingdom. $\endgroup$ – Li Jun Oct 13 '20 at 8:45