I don't think Ming dominance would have prevented European imperialism. While the Chinese dynasties demanded submission and wasn't really into European-style imperialism (at least if you submitted peacefully, unlike the Dzungars), the flip side was that they didn't do much to help you. Malacca paid pretty regular tributes to the Ming all throughout the 15th Century but when Vietnam tried to invade them in 1471 China's response was essentially: 'Um, sorry guys'. And of course China didn't do much when the Portuguese actually took over Malacca. Korea was a major exception because it was considered the first among vassals, with Vietnam/Annam (which the Qing would go to war over for in the 1880s) a distant second.
I think you really have to screw over the European states for European imperialism to not take off. England, France, Spain, Portugal (and later Scandinavia and the HRE) must simply become hubs of endemic warfare, which means that even if the rest of the world is 'discovered' by intrepid adventurers, there isn't the capital or the inclination to exploit these openings. Maybe a failure of European monarchs to centralize, and the consequent splitting of their realms under feudal rule, could contribute to this.
Of course, you could make the native Americans/Africans/Asians stronger as well, but it's hard to see how development in those areas doesn't eventually rebound back to Europe.