Taizong Huangdi (Li Shimin) doesn't get killed and goes on to be one of the greatest rulers in all of chinese history since the primordial times to the moderne ones. He forces the eastern and western turks into submission, and the tibetans; his whole territory losely looks like a giant phallus on the map, and the silk road is the pi** His Majesty directs at Europe. 
As for Ashina Sun & Chang'ge, I don't know, maybe they will elope westward to happily annoy the greeks and the muslims everafter, or maybe they died somewhere forcing the Emperor to ask Qin Qiong Shubao & Yuchi Gong Jingde to guard his chamber door against the spirit of butthurtness. 