I'm not quite sure if this'll get much in the way of response, but here goes.
Having gotten much more interested in this period of Chinese history, a somewhat large WI came to mind. What if Cao Cao, having already united the North China Plains and on the verge of crushing all opposition, been victorious at the Battle of Red Cliffs? Butterflies away the Three Kingdoms era, certainly, but what are the long term effects of such a victory (and long-term effects it might very well have)?
Regretfully, the battle of Chibi was begun as an overlook in strategy on parts of Cao Cao. Its success would mostly not see a "finish-off" of the coalition and quick unification of all China, but rather a orderly fallback, reorganization and reconsideration of strategical position. Cao Cao likely hadn't predicted a full-scale conflict with Wu at the beginning of the Jingxiang campaign. He didn't really have that much manpower to utilize from the unified but devastated northern plains to completely overpower a considerable number of better-trained and adapted Wu army, estimated at 30 thousand.
As other fellow members mentioned, the Wei army was consisted of many newly-surrendered, demoralized soldiers of former warlords, Cao Cao was not really at a position to advance into the heartland of Wu even if Chibi was a victory. And further long-term development still had to depend on the reactions and measures taken by the three sides, although with Cao Cao being in a generally better shape than IOTL.
Strategically, given his careful approach, it seems that he had wanted to extinguish any viable forces of Liu Bei before it turned into a serious threat (which was later proven correct), and conquer Jingxiang as a base to train a water-capable army, and only then to deal with Wu. He succeeded in conquering Jingxiang but fail to capture Liu Bei. Cao Cao then made a wrong assumption that Wu would not immediately try to lend a helping hand to and ally with Liu Bei, but go for its own survival at times of severe threat.
He thought he can destroy Liu Bei's army before Wu decided to help, thus he ordered his under-prepared army to chase after Liu Bei and sent three regiments to the east front to seemingly wage a attack on Wu (plus an envoy to Wu who greatly exaggerated the number of his force), in order to scare them into submission, or at least make them concentrate their force instead of sending out their army first. It didn't work as what Cao Cao would have like, and Cao Cao was forced to fight a war with his army diverted, the 70 thousand soldier formerly belonging to Liu Cong as the bulk of his army, and with widespread blood-fluke infestation to add salt to the wound; Careful and calculating as he was, he definitely wouldn't have been pushing his luck again even if he had won there, which would have been quite a miracle and at a great cost in any situation.
Some say if Guo Jia hadn't died at such an early age, he could somehow magically turn this big defeat into a victory. But IMO, given his strategical insight, he would have dissuaded Cao Cao from launching this under-prepared campaign to begin with.
Wei's still going to have problems further down the line if we assume the same succession. The Cao Cao's descendants were a pretty useless bunch.
I can imagine Wei not lasting too long. Strange how often some of the smartest people in history have absolute pillocks for descendants.
(snip)
Well, at least his immediate descendants weren't that bad. Cao Ang was humble and competent, yet died tragically early. Cao Pi was also a competent administrator and innovating legislator, though he was not magnanimous and fair enough as his father, which may have leaded to the intrigue later in the court; IMO his initial indecisiveness in choosing a heir may have also deprived Cao Rui from the chance of a proper heir-grooming, and everything went downhill thereafter.