Getting back Taiwan and Korea, as well as everything else belonging to the Qing Empire is the first step, obviously. If the Xinhai Revolution comes fifteen years early (after the FSJW) or even a decade in advance of the OTL date, that would give the RoC more time to establish itself before Russia has its revolution in 1917 that makes Communism into a new fad. There would still be leftist opposition, but a KMT with more time to build itself would have less problems dealing with the CCP which would only come in 1921. With greater political stability, the KMT would've gotten a head start in modernization of industry, military and the society. This could have the following effects on China's international status.
- China might have come to butt heads with Japan again in the 1910s or 20s, and if they win, we could see the first of Republican China's territorial conquests in the form of Korea and/or Taiwan.
- During the Russian Revolution, the KMT might have taken the opportunity to seize the Outer Manchuria territories owned by the former Russian empire. For this purpose, they could either be enemies of or indifferent to the Bolsheviks, if the former, they could claim to be helping get rid of White forces, which had a big presence in Siberia during the RCW. If Japan gets involved in the Russian Revolution like OTL, this Japanese presence as well as a Chinese one could lead to the aforementioned war with Japan in which Japan is defeated. End result? China gets its Siberian/outer Manchurian territories back, as well as Mongolia.
- In 1919, instead of being ignored by the major powers at the Versailles treaty, China could get more recognition as an emerging power. To appease it, the Western powers might give Qingdao back to China instead of Japan.
- Relations with Soviet Russia would be cool at best, given that the KMT has just taken a big chunk of Siberia, and Japan would be China's biggest enemy. If they still have Taiwan, this could prove a big point for conflict. If China plays its cards right, it could have good relations with the Anglosphere, and do a great deal of trade with them, enjoying a status mirroring that of the modern PrC. Like in OTL, they could get military tech and know-how from Weimar Germany.
- As China is more of a power than in OTL, Germany wouldn't break up with it in favor of Japan, and the Sino-German cooperation would continue. As a result, as TTL's WW2 nears, perhaps China would attack Russia. My guess is that they would lose (as Japan did in 1939), and focus their strategic bulk toward Japan anyhow.
As TTL's WW2 draws near, we would see Japan standing alone and increasingly desperate as China gets stronger and stronger under KMT rule and with foreign help. TTL's "pearl harbor" attack may very well come in the form of a massive Japanese attempt to sink the entire Chinese fleet in port, while making lightening attacks on Korea, Hainan, various ports, and as in OTL, the oil-rich DEI. They might bypass the Philippines for fear of angering America, which ITTL has been replaced by China as Japan's archenemy. In any case, TTL's Pacific War will be a showdown between China and Japan, the two modernized powers of East Asia.
- The war in the West, depending on butterflies, might give the Nazis more luck, enough to defeat Russia and go on a path ending in either the "Greater German Reich" scenario or CalBear's TL, or give the Nazis a harder time in key areas like Poland or France. Either way, Germany would have no reason to quit being friendly with China, and would continue helping it up until either it has no resources left to spare or war conditions make this impossible.
- China's situation with Russia is uncertain, but I think the best bet is an uneasy peace, similar to what Japan had. If Germany gets the lucky butterflies and conquers European Russia, expect China to go and swallow the rest of Siberia in the later stages of WW2. This however is not that likely.
- The war with Japan could go a few ways:
-- China kicks ass from the get-go and brushes off Japan's surprise attack. No Chinese territory is conquered, Japan forced to give up everything but the home islands.
-- China is briefly shocked by the blitzkrieg, and Japan succeeds in taking Korea, the DEI, and severely damages the Chinese navy/air force. The war goes on until China wears down Japan with attrition, culminating in either a favorable-to-China peace settlement or invasion of Japan, after which Japan becomes a Chinese puppet or grudging ally.
-- China suffers in the initial attack, and the Japanese are even able to take a lot of territory on the mainland, though not as much as in the OTL SSJW. However, China is able to crush the Japanese in several major battles, driving them off the mainland. If America and the UK get involved, it could end in Chinese and Allied occupation zones of Japan. Given the probable lack of major ideological disagreement between China and the US and UK, Japan would eventually be made independent and peaceful.
Thus, after TTL's WW2 (and 3rd Sino-Japanese War), China would be a rising Great Power, at least on par with Russia (assuming Russia defeated the Germans), with Japan having been decisively defeated as a militant power in the war. It would be for sure a great military power, and with economic cooperation with the Western Allies, have a burgeoning industry as well. By 1975, I could see China having the world's biggest economy.
and evacuate the mainland.