Another argument against going beyond China's OTL traditional borders is a lack of need. To the north there wasn't that much in the way of goods that Chinese citizens could not make or grow themselves, rendering the only reason why successive Chinese armies conquered or fought the steppe tribes there being prestige, which was quite expensive, or defence, which was necessary. The same situation pervaded in the west, in Sinkiang and Central Asia. If you're an Emperor and going to bring those places under heel, and have the excuse to do so, you're also eventually going to have to contend with distance and the usefulness of the place under direct rule combined. From what I know from histories of Genghis Khan, Central Asia was a veritable trading hub and advanced centre of civilisation just before the time he went around and killed everyone, and it was relatively the same under Tang rule. But again, garrisons are hard to maintain all the way out there and frowned upon anyway, which'll eventually make even the holding of places like Sinkiang increasingly hard to justify. Not that these areas weren't conquered; the Han, the Tang, the Yuan and the Qing all did so, but other dynasties like the Song, the Liao, the Ming, did not. Some of that was because of massive internal and external pressures, and some because there wasn't that much of a need; the tributary system satisfied. To the south lies South-east Asia, which looks a lot better; then again, Vietnam has a history of fighting very very well a long time before the Americans arrived. The proud martial history of the Vietnamese stretches back all the way to the first Chinese invasions, which inflicted so many casualties on successive armies that the region was included by the Emperor Hongwu (i think) on a list of countries to steer clear from..