Tokugawa Liang wrote:though Sun Quan founded a new dynasty, it was still part of the Han pire, what was the interest of expanding south?
I doubt Sun Quan viewed Wu as part of the Han dominion, especially since he'd formally established the dynasty after Han's formal dissolution. That said, I doubt he ever had any real loyalty to Han even when he was nominally a subject of the Emperor. Sun Ce likely felt the same way, and so did Sun Jian, at least after the breakup of the coalition.
Wu probably expanded south for the same reasons Russia expanded east, though more dangerous rivals lay in the opposite direction. As you said, it was often easier to conquer. I'd bet many conquests weren't planned campaigns. For example, a Yue raid on a Wu village would prompt Wu's retaliation, culminating in the annexation of the Yue land, and an influx of Han Chinese settlers.
The South was also relatively undeveloped territory with massive potential. Look at China today-the richest province is not Henan, where Wei was centered, but Guangzhou, where Wu spent much of its existence developing. Zhejiang and Fujian are also rich provinces, and those regions have, I believe, been relatively wealthy ever since Han Chinese settled there.
Wu had to develop these regions for the sake of its economy. Can't fight without money, and you can't get money without resources and the ability to support a large population. They developed the South for the same reasons America developed the West. And they succeeded-the south before Wu was a backwood. After, it often eclipsed the North as the political, economic and cultural center of Chinese civilization. Not too long after Jin united the kingdoms, it was invaded by the Wu Hu. Jin survived only by retreating south of the Chang Jiang (Yangtze), and only because Wu had developed the land enough for them to rebuild.
Trade was also important. Expanding south gave Wu the chance to seize control of the maritime trade routes connecting China with India, Persia, and Rome. It's possible that this could've even them a leg up on Wei, as the land route was more contentious and less controllable.
So in short, Wu did it for the money, or acted as an empire acts.
I believe de Crespigny's
Generals of the South might have more info you might like, as might this section of the
Weilue (with added commentary) I posted on this thread:
viewtopic.php?f=5&t=21573&p=569644#p569644 I'll warn you, though, the bastard translator used Wade-Giles. The link at the top of my post over on that thread has all the surviving
Weilue, I believe.
I don't write fanfic, but if I did it would involve Zhou Yu and Zheng He fighting to win the heart of Lai Choi San. Then I'd make them join forces to fight Ming the Merciless, who secretly works for Master Li. I'd squeeze Lu Bu in there somehow.