Japan would probably not have had Manchukuo and China unite. After all, for decades, Japan tried to somewhat "rewrite" the history of the Manchurian region. We can see this even in Inaba Kunzan's 1918 work's introduction (by another writer) - This introduction talks about Manchuria's history with the Khitans, the Jurchen, and the Manchu, but mentions the "Chinese" (Han) as some sort of outsider that, only recently, disrupted the historical path of Manchuria. We can see that Manchuria was set up from the start to be an independent (or, while not independent, autonomous) state under the Japanese. We can also see this in Japan's forcing of Manchukuo's citizens to take up the identity of "Manchu", and in the Koumin, the 1940 text used to train police officers - It celebrates the closeness of Manchuria to Japan (the Koguryo period and Manchuria's relationship with Heian and Nara Japan) and the Khitan and Liao ancestry of the Manchus. It does not even mention the Chinese at all.
Then we get on to Puyi (Datong/Kangde). Puyi served his job as a puppet. Throughout the course of Manchukuo's existence, Puyi had been made increasingly subordinate to the Japanese emperor. In 1940, he was even ritually reborn from the womb of Amaterasu, thus making him Emperor Showa's brother. His use was to make the Chinese into increasingly servile subjects, and using him worked.
There were then the other parts of China, under the Wang Jingwei Regime, which, initially, had even been split up. While Manchukuo was seen by the Japanese as a completely legitimate government, an empire grounded in history, Wang Jingwei's government was recognized by the Japanese as the legitimate government of China (not including Manchuria due to reasons stated above). Towards the end of the war, it was pretty much clear that China would at least have been split into two.
Japan's aims in China were not to have a strong China, either - Their main aims were simply economic. Divide and Conquer would haev been a very good tactic for them; They wouldn't lose much to achieve their goals of a massive captive market, a strong source of natural resources, and opportunity for Japanese corporate and political power to expand.