Kill off Cai E before he could leave Beijing. Without Cai E, the revolutionary and progressive forces in the South won't unite that fast. They would eventually find another leader to rally behind. When that happens, Yuan is still doomed to fail as not even Beiyang military personnels agreed with Yuan's decision to make himself Emperor. He may hang on for a year before the South unites and the stress kills him off, but once he dies, the monarchy would be over. His son simply could not control the military. Duan Qirui would assume power in a coup on the next day.