Probaly get absorbed by Russia; without the Qing uniting them with China, it's very likely that they'd meet the fate of similar Khanates and Siberian Tribes and simply fall victim to Russian Imperialism. They could unite and maybe remain independent like Mongolia, but it would likely have the relevance of Modern Mongolia and even then its hard to see them not get eaten by Russia and if not Russia then by Japan. If they retain their identity until the Communist Era, they could become a consitutant part of the USSR(Thus technicly remaining independent) and then declare independence like other Communist Republics. The Ming would still exist, or perhaps the shortly lived Shun Dynasty would have taken over. It could be good for China, as Li Zechang might tend to establish a reformist China thanks to his roots, political stances and Non-Han Ethnicity(Yet still Chinese, unlike the Manchurians). And since the Qing conquests would never happen the Uyghurs might be independent or part of Russia whereas Tibet would likely be independent aswell. Perhaps Manchurian Russia could lead to a much longer and more bloody Russo-Japanese War? Really, the potential is endless but I feel like the lack of Qing would, atleast in the Modern Era, weaken China overall due to the fact it would be restrained to China proper although they may fair better during the age of European Imperialism where Qing suffered through numerous conflicts.
Best chance for a whole Manchuria: They get conquered wholesale by the Russians, outer manchuria and all. They declare independence during the Russian Civil War before a communist coup takes power. They then join the USSR and gain independence during the fall of the Soviet Union whilst still having Outer Manchuria.