...and my opinion on my own topic. I think both Guo Jia and Fa Zheng could have changed events drastically.
Guo Jia counseled Cao from his rise from a small lord to the mightiest lord on the continent. Zhou Yu's victory relied on the success of several ploys such as the use of the brothers Cai and the exposure of Cai Mao and Zhang Yun. Cao had no skilled advisor that could see through these plots, relatively simple as they were. A naval commander the capacity of Cai Mao would have aided Wei as they battled the Wu fleet. Guo Jia would have certainly also assured that Wu would not simply be able to walk over the Wei rabble to victory in battle, and it is likely he would have had movements of his own to counter Wu's naval strength.
Fa Zheng had arguably an easy victory at Han Zhong, but it was more likely not so. Han Zhong was a well-fortified city, defended by one of Wei's greatest generals, known for his skills and wisdom in battle. Zhang He aided Xiahou's defense, and Wei had its constantly-present material advantage over the comparably minute Shu army. Yet Fa Zheng guided Shu's army to victory through use of the valiant generals Yan Yan and Huang Zhong. Several Xiahou clan members fell there, and when Cao Cao brought his massive reinforcements, Fa continued on to destroy them, and very nearly Cao himself had his son, the great general Cao Zhang, not saved him. His offensive strategic capabilities were clearly some of the greatest in the Three Kingdoms, and judging by the destruction of several Wu armies before Yi Ling, Fa Zheng would just make Yi Ling the icing on the cake. Dingjun showed that Fa favored having terrain advantage, something Ma Liang did not seem to care about when he allowed Liu Bei's massive army to encamp in a dry forest in the heat of summer. Either Fa would have countered the strategy in such a way, or would have pressed the offensive and hit Lu Xun with Shu's greatest strength - valiant generals in battle. It destroyed Wei easily at Dingjun, and Wu was even less well-prepared then Wei to face a strong Shu onslaught as Shu's early victories showed. No Wu army could stop the fury of the Shu army in the campaign for revenge, and Fa Zheng would have assured this at Yi Ling
"I have gained this by philosophy: that I do without being commanded, what others do only from fear of the law." -Aristotle