They should just retake outer mongolia first.Despite US stagnating economically, I still believe the US is the largest civilisational threat to China in all 5000 years of history. It's the only country that has good enough geography to match China so far. The huge amounts of farmland and rivers gives the US the ability to sustain a large population. Even though they have barely 1/4 of China's population today, nobody knows what will happen in 2,3,500 years.
For China to remain number one it needs to expand. North to Siberia and south to Vietnam. Maybe not now, not even in a hundred years but it has to happen. Unless the US implodes into different cultures like Europe, but that's unlikely as technology has standardised language.