The later Wei wouldn't be built. And I assume that "Wei" was no longer a part of the three kingdoms. It would just be the Han. And maybe Wu...

What if Nobunaga defeated Mitsuhide at Honnouji and survived?

I think Nobunaga would have frightened all other daimyos in Japan into submission and he would undoubtedly rule Japan, and who knows, he might have moved on to China....

what if Nobunaga's father hadnt died untimely??

Then perhaps Nobunaga would never have gotten to were he was, or he and his father would have amassed an even stronger army. Hard to Say.

What if Wei had continued to work for Orochi?

I think that initially, the coalition forces would have been pushed back due to the extra force Orochi would have had but eventually I think relationships such as those between Cao Pi and Da Ji would have broken down so badly that I think a lot of in-fighting would have happened, possibly resulting in a lot of the Wei forces being destroyed if the coalition forces weren't around to give them aid

What if Da Ji had discovered Zhuge Liangs false loyalty towards her?

He may have been defeated at that point, lead in to a trap. Either that or he would have something prepared for that eventuality.

What if Masamune wasn't working for orochi?

Hmm well I suppose the coalition forces would have a new ally...although I'm not sure he would be very loyal to them. I don't think it would make much difference to Orochi's forces but perhaps if Masamune was persuaded the help out by Magoichi or someone then he could make a bigger difference against the snake forces

What if Hanzo killed Zhou Tai at He Fei rather than just putting his blade to his neck?

Ummm.... Zhou Tai would not join Wu. Further more, Sun Quan would refuse to join Sun Ce and he joined other forces. Or... Sun Ce and Sun Quan would end up killing each other and Sun Quan would sent someone to assassinate Hanzo.

What if Pang De succeeded killing Guan Yu in Fan Castle?

Omg that would have changed so much! Liu Bei wouldn't have wanted revenge on Wu (as much) so Yi Ling wouldn't have happened so then perhaps with extra persuasion from Zhuge Liang, Liu Bei would have formed a truce with Wu and attacked Wei together. Zhang Fei would still have gone mad and got himself killed and I don't think that Liu Bei would live too long after that but still, Wei could just possibly have been destroyed if that happened Oh and Pang De would have been made into a playable character much more early on in the DW series for his massive achievement

What if Sun Jian wasn't killed in the rockslide when he invaded Jing?

Wu would have probably been led better, but then again Sun Ce was a great leader too. Who knows? Sun Jian might have led Wu to victory.

What if Lu Bu had defeated Cao Cao at Xia Pi?

(This hasn't been asked before has it?)

It would change everything. When Cao Cao defeated, Liu Bei would be also defeated. If they two were killed, there weren't any Wei and Shu. Maybe just Wu and Lu Bu (in the other word, it won't be Three Kingdoms anymore...)

What if Dong Zhuo and Lu Bu knew about Diao Chan's and Wang Yun's plan?

Dong Zhuo would have tried to have had Wang Yun executed and Diao Chan punished somehow...but I think that Lu Bu would still have stuck with Diao Chan through total infatuation so would probably have struck Dong Zhuo down when he gave the order to harm Diao Chan...without planning though, Lu Bu would then probably have been killed before he could have escaped with the officers still being loyal to Dong Zhuo rather than having been persuaded by Lu Bu to join his cause or anything

What if Liu Bei had an affair with Yue Ying?

Zhuge Liang would have popped out of the closet or somethin while they were in the act and beat them with his fan!

what if Lu Bu had a son?!

That's an interesting one. I think Cao Cao might have taken him into his care after Xia Pi and brought him up as his own son, hoping to mould him into a supreme warrior only loyal to him (or something like that )

What if Jiang Wei and Deng Ai had defeated the forces of Jin?

If they were sensible and put their differences aside I think that they would have soon invaded and overpowered Wu, being in such a fragile state. Then (especially if they decided to get rid of Liu Shan) I think they may have lived quite nicely - Jiang Wei following Zhuge's teachings and all and with Deng Ai by his side, relations with people from the original Wei area would have been good so yah - I think it perhaps would have bought peace

What if Zhang Liao had joined Guan Yu when he left Cao Cao for Liu Bei?

If that had happened, a few years down the line Wei would probably have been defeated at He Fei and been weakened. In turn, that means Wei and Wu prolly wouldn't have allied at Fan Castle, meaning Guan Yu would have lived longer. Shu and Wu would have allied to destroyed Wei, and then fought between themselves.

What if Sun Jian never found the Imperial Seal?