I think that Liu Bei would have had a very tough childhood, most likely being picked on by Cao, so wouldn't have had the confidence to become an influencing leader in the first place Cao Cao would've just carried on as normal, uncaring

What would have happened if Ma Teng + sons weren't assasinated by Cao Cao?

Then, Ma Chao would have never joined Shu and Cao Cao would have less power. Also, there would be a war against Cao Cao and Ma Teng.

What if Zhuge Liang never died?

Other than there being an extremely creepy old man wandering around, I expect that Shu would have eventually overcame Jin, with Deng Ai, although being brilliant, not being able to compete with Zhuge in terms of strategic ability. Then Shu would have probably allied with Wu if Zhuge held his power over Liu Shan etc.
And then he would of cause have easily broken the world record for oldest human, still going at the grand age of over 1700 or something and I'm sure would have made a strong impact on the medical and technological world

What would have happened if Gongsun Zan had destroyed the forces of Yuan Shao?

Then, Gongsun Zan would have controlled the North, most likely, and would be in competition for power, with Wei.

What if Mitsuhide won the Battle of Yamazaki and not Hideyoshi?

Then istead of Mitsuhide being the "thirteen day shogun" he would have been closer to the "twenty day shogun" because Ieyasu still had plans for ruling.

What if Shingen and Kenshin never died of illness?

Haha that would be amusing:

Liu Bei: We are brothers??!!

Cao Cao: If I must kill my own family to rule the land then so be it..

Liu Bei: I cannot fight another member of my family, I surrender..

Cao Cao: Wow that was easy, how about a game of Go?

Liu Bei: Sure

Yeah I was really bored.

What if Guan Yu hadn't escaped from Cao Cao after the battle of Bai Ma (I think I've got the name right)

The I think he would have fought to the death - he'd said explicitely that when he knew where Liu Bei was, he would go straight to him and certainly wouldn't serve under anybody else. I don't think even Zhang Liao could have persuaded him otherwise a second time...

What if Xu Shu had been able to stay with Liu Bei?

sorry about that before people, accordin to my comp there wasnt even a second page

Shingen & Kenshin would have probably continued their fight until one of them was destroyed. No idea who would win tho

Xu Shu was said to be an amazing strategist, not sure about in comparison to Zhuge Liang though, if he had stayed then Liu Bei would have had a much better chance unifying China.

What if Guan Yu had managed to fight his way out of Mai Castle?

Its fine Spear master. imjust glad someone finally answered mine.

If Guan Yu had fought out of Mai Castle, he would have been able to launch a counterattack with his army and might have razed the castle.

what if the fire attack at Chi Bi wasnt successful??

That would have been a very quick battle.

What if Wu had taken He Fei?

Then, Wei probably would have been crushed or forces to retreat somewhere else. That was his base of operations, essentially and a huge importance to Wei.

What if Dong Zhuo won the Battle of Hu Lao Gate (and Lu Bu didn't betray him later on).

He probably would have lived off the glory for a while, maybe had a stronger base of power, but undoubtedly would have been defeated somewere down the line.

What if Nobody beat Orochi?

Oh, hi BG, welcome back. Hope you had a nice time at your prom. We didn't do anything bad, honest.

If nobody defeated Orochi...I think the Coalition army would have been torn apart and they could have been destroyed.

What if Katsuyori Takeda won the Battle of Nagashino and the Takeda Calvery was still intact?

I think he would have been defeated somewhere along the lines, if not by Nobunaga then maybe Hideyoshi after him.

What if Lu Bu had defeated all three sworn brothers at Hu Lao Gate?

There are no Shu, then. And maybe it's not Three Kingdoms anymore...

What if Yuan Shao killed Cao Cao in Guan Du?