Let's imagine that Li Zicheng had been a bit more successful against the Qing as well as the Southern Ming and can survive at least a century (ideally several centuries). He manages to carve out a state like
depicted on this map but including Northern China up to the mountains near Beijing. North and east of him are the Qing, licking their wounds from their defeat the hands of the Shun, and the Shun conquer up to the Qinling Huaihe line in the south. Zhang Xianzhong is still active in Sichuan, and the Southern Ming are reorganising to face these challenges. The Shun want to reunify China and expel the Manchu barbarians as well as protect against other threats like the Oirats in the west.
So what should Li and his successors do about the capital? Initially Xi'an was his capital, which is a fine capital for a ruler concerned about legitimacy, but the terrain and challenges of supplying Xi'an make it a poor choice for the long run. Beijing is too close to the frontier IMO, and too far from Southern China which is an economic center and one which the Shun will want to conquer. This would leave the best choices being Kaifeng and Luoyang.
OTL Kaifeng was flooded (through destroying flood protection) in 1642 by Ming authorities to slow down the advance of Li's army. This apparently wrecked the city, so Kaifeng would not be a suitable choice immediately. However, let's assume the local authorities decided not to do that, so Kaifeng remains unflooded and thus a prosperous local city with a suitable tradition. The main problem with Kaifeng at this point becomes the need to defend it, as Kaifeng is challenging to defend against attacks from the north. However, it's wealthy and centrally located within China, and far from the frontier, so long as the Shun can hold Beijing and the local military and civilian rulers in Beijing remain loyal to the Shun Emperor.
The second choice is Luoyang. This is more inland and rather more protected than Kaifeng, but Luoyang was only a regional center in this era despite it's grand history. Luoyang gives protection to the Imperial Court from all foes, and perhaps can focus China's power west, so the Oirats, Mongols, etc. can be subdued. Luoyang is even more centrally located in China than Kaifeng, so it could be a very suitable capital indeed.
Which capital would be best for the Shun given their immediate circumstances? Should they initially go for an Eastern/Western capital scheme, where the government alternates between Kaifeng and Luoyang before permanently relocating to Luoyang for whatever reason (probably simply laziness and the need to not spend the money to relocate so many people)? I'm leaning toward Kaifeng, but Luoyang offers some advantages too.
Thoughts?