One thing that isn't often speculated is a figure other than Liang becoming the figurehead of the rebellion. There were numerous bandit armies and even small kingdoms that were declared and which operated during the general phase of the black turban disorder. Most threw in their lot with the Jianshun Emperor after Great Cai was declared but some had to be exterminated, like Ning Shaofeng's Great Chen state that was a major force in guangdong, or the Great Shu regime that was founded by Wang Liao in Sichuan. Great Cai won out primarily due to luck in geography, being a massive central and northern revolt that blocked Qing access to the south and presented a far more formidable threat to Beijing, while the ancient capitals of Chang'an, Luoyang, and Kaifeng practically fell right in the Jianshun Emperor's lap (incidentally resulting in the emperor setting up his permanent seat in Luoyang and revitalizing the whole central region)
What if Liang's rebellion hadn't been as strong and Chen or Shu had been the major unifiers of Chinese resistance, resulting in Qing having a better hold on the north and a better defensive position, and out of this a Northern and Southern Dynasties period begins? Would the Europeans have taken a side, or perhaps even played both against each other? Would modernization actually have accelerated due to constant war on the frontiers? How would the Japanese have reacted to this?