Historically, Emperor Gaozu thought about naming Li shimin his successor in 621 itself, however the concubines who favored Li jiancheng changed his mind. ITTL this doesn't happen. Li Yuanji had been banking on Jiancheng being named crown prince so that when Jiancheng ascended the throne, he himself would become the crown prince. Historically, Li Yuanji was offered the hand of Bhrikuti Devi by Nepal and King Amshuverma, however with Li Yuanji's ambition for the throne that didn't happen otl. ITTL with the thrown decisively out of reach, and his power base about to die, Li Yuanji takes the offer. And history changes.