I think the following wou happen:
1. Cao Cao's force would probably still hold together. Cao Pi was alive and old enought to rule, so most Cao officers would stick together around Cao pi.
2. With Cao Cao dead, most likely Cao Pi would want to conclude a peace treaty to mourn for his day (Much like when Sun Jain died)
3.Yuan Shao, being the senile old idiot he is, will accept the peace offer (like Liu Biao)
4.Cao Pi would now consolidate his rule in Wei, with the help of Cao's old officers. (Like Xun Yu/Cheng Yu)
5.I doubt Cao Pi would let go of Guan Yu. Cao Pi unlike Cao Cao did not admire him. But I would think that Guan Yu would be able to escape somehow...
6.While Yuan and Cao are fighting, Liu would escape with Guan to Ru Nan where Zhang is. There, he would gather followers and power. Back in the north, I doubt Cao Pi could get any advantage
7.liu Biao would soon die, leaving Jing open for Liu Bei to take. Sun Quan and liu Bei would get into a fight over Jing.
8.Yuan Shao would die of old age, and Yuan Shang would take over. While Tan and Shang fight, Cao Pi would take over.
9.Liu Bei would take over Jing, because that he is "related" to Liu Biao. Now I'm thinking in Jing, Pang Tong/Xu Shu would join Liu Bei, LIu Bei may be able to take Yi from Liu Zhang....Ma Teng of Liang would also joing Liu Bei also because they were friends once...
Now "three Kingdom" situation would again occur, With Liu Bei holding Jing/Lian/Yi, and Sun Quan holding Yang, and with Cao Pi with teh rest.
10.Liu Bei would take over, with Liu Biao/Ma Teng's officers and armies. Liu Biao's navy can be put to use against Sun Quan, and the Liang cavalry can mount an offense against the central plains. Soon, with Sun Quan subdued and the central plains conquered, Liu Bei would rule supreme.