Note: The first few posts would be primarily to outline ideas.
POD: Liu Bei doesn't drop his son on his head like a dumbass. Thus, Zhuge Liang doesn't work himself to death (from having to perform the duties of Liu Shan and himself) and thus lives another 20 years. Ultimately, the goal is to have the Kingdom of Shu establish a dynasty in China. This might be difficult because of the lack of manpower and food supplies that Shu had versus Wu and Wei.
There will be two rather improbable events that occur before Liu Shan comes of age and assumes his father's duties, in order to make things easier for Shu.
1.) Guan Yu holds Jingzhou and Fancheng. This would be important for increasing Shu's population base.
2.) Somehow, that temporary alliance between Wei and Wu has to not hold up, despite the loss of Jingzhou.