The Crown Prince, a term brimming with feudal characteristics.
As the Emperor's son, throughout history, the Crown Prince and the Emperor have always been a naturally conflicted duo. Apart from a few exemplary father-son pairs, many relationships between Crown Princes and Emperors aren't particularly good, and there are significant conflicts.
The Crown Prince wants to become Emperor; the Emperor doesn't want to give up power. It's that simple.
But Emperor Hongxi is different.
Emperor Hongxi's Crown Prince Zhu Zhanji is the eldest son of the current Da Ming Empress Zhang. The title of the legitimate eldest son is embodied in him to the fullest. It can be said, unless Emperor Taizu Zhu Yuanzhang himself returns through Impure World Reincarnation, no one has a more legitimate claim to the next Emperor than Zhu Zhanji.