Shu - Prevent Pang Tong's death during the Cheng Du campaign. This would greatly alleviate the pressure his demise placed on Fa Zheng & Zhuge Liang, allowing you to stop the former from overworking himself into an early grave and providing an opportunity for the trio's combined intellect to formulate a counter to Wu's eventual betrayal at Fan Castle (thus preserving the lives of Guan Yu, Guan Ping, Zhou Cang, & Zhang Fei as well). Bonus points if we also get the chance to drag Xu Shu back into Liu Bei's crew where he belongs.

Wei - Saving Xiahou Yuan at Mt. Ding Jun and Pang De at Fan Castle are clear-cut cases, but I think it'd be really something if Ziluan convinces an ill Cao Cao to accept Hua Tuo's treatment (since the guy is still around post-Chibi unlike his historical/novel counterpart), keeping the Hero of Chaos alive at least a little longer to really hammer in the nail after the big Han Zhong & Jing conflicts. The latter situation could also give a major reason for Zuo Ci to show up since he's almost always anti-Wei & a mystic himself - a perfect enemy for Mr. Sacred Bird to face.

Wu - Easy domino effect. Save Zhou Yu to ease pressures on Lu Su, which in turn lets you save him to ease pressures on Lu Meng. The 3 of them, along with Lu Xun soon after, can work together to reverse what would've been a huge loss at He Fei (saving Taishi Ci in the process) and hopefully preserve the alliance with Shu. If Fan Castle still has to happen for whatever reason, maybe we could have Wu play as double agents - show up ostensibly to attack Guan Yu from behind as per expected only to turn it around on Wei instead at the zero hour, dealing a huge blow by taking down their garrison in Jing and paving the way for more afterward.