Perhaps a stronger Tang China (no An Lushan rebellion?) forms an alliance on equal grounds with the Uighurs against either the Tibetans or maybe a more belligerent Abbasid Empire?
The An Shi Rebellion could be averted, but other generals not under the government's direct control will continue to maintain significant influence within far-flung regions, making it difficult to prevent a major uprising for long. For example, Li Zhengji (Yi Jeong-gi), a general of Goguryeo descent, managed to govern most of Shandong with over 100,000 troops, many of whom were also of Goguryeo descent, and took advantage of the chaos caused by An Lushan by technically proclaiming his own dynasty, which was tacitly acknowledged by the court in Chang'an. Although he remained subordinate to the Tang, his successors continued to remain autonomous for decades afterward.
Culturally, if you want cultural ties between states on the steppe and the Chinese, I think it would be easier for the Mongols/Uighurs/Gokturks/Xiongnu/Xianbei to adopt Confucianism than for the Chinese to adopt steppe practices. Granted, Confucianism arose in a cultural milieu quite different from the steppe environment, but the same can be said for Buddhism, which was accepted.
Buddhism tended to be combined with other beliefs, such as shamanism. Although Confucianism was also promoted within China in conjunction with Legalism as well, it tended to be more rigid in regards to general policies due to close ties with Chinese culture, meaning that it was easier to rearrange Buddhist ideology in order to fit each region/state.
Nomads and Sedentary civilization having good relationship and founding united state is hard. However there was attempt to create coherent nomadic-sedentary state.
One example is Balhea. If I remember correct Balhea was not genuine sedentary state. It was combination of Gogorye founded by Gogorye general who allied itself with Jurchen/Mohe/Malgal nomad tribes. I don't know how demographic structure was, but it should be fairly equal between nomads and Gogorye people. It functioned as fairly stable state.
Another attempt is Liao Dynasty. The Empire was founded by Nomadic Khitan people. After conquest of Northern China, first Emperor of Liao tried to establish stable Nomadic-Chinese Empire.
From wikipedia.
In that sense, Goguryeo was also a nomadic-sedentary state, as it frequently allied with Xianbei and Mohe (Malgal) tribes as well, as Chinese and Korean records both state that the latter two frequently participated with the former during conflicts within the peninsula and against the Sui and Tang. However, both Balhae and Goguryeo were limited to Southern Manchuria and the Northern Korean Peninsula, so it would be difficult to adopt their models to a state containing significant Chinese and nomadic populations.
The Liao provided a reasonable model for governance, but its policies would also probably be difficult to implement within a state with a much larger territory and population.