I have a few suggestions for characters you can add. If more information is required or input needed on possible promotions, I would be happy to help with that. For now it's kind of late and I just thought I'd jot down some ideas for fleshing out some of the lesser factions. I realize that balance may be an issue here. I'm not sure how well some of these characters would work out in light of this.
Yan Xing (Han Sui loyal): According to Dr. Rafe de Crespigny's
A Biographical Dictionary of the Later-Han to the Three Kingdoms (from now on simply called "Rafe's tome"), he was a strong officer of Han Sui's. When Han Sui and Ma Teng had a falling out, Yan Xing dueled with, injured and nearly killed Ma Chao in AD 197. Yan Xing urged Han Sui to engage in diplomatic arrangements in Cao Cao, advice that Han Sui indeed followed until joining with Ma Chao in opposition. Later Yan Xing betrayed Han Sui in a surprise attack, but was ultimately forced back. He served Cao Cao thereafter. Crespigny cites Sanguozhi as his source. Perhaps he could have high cavalry potential like Han Sui, since that seems to be that force's focus.
Gongsun Du himself (Gongsun Du): Gongsun Du was actually called Gongsun Du The Warlike (even in the novel, where he is only very briefly mentioned) because of his talent in warfare. When I played him, the faction only had Liu Yi as an officer, however. Again keeping with the focus of that force's strengths, I'd say he should be a largely naval officer, but perhaps with skill in leading melee troops as well so as to compliment Liu Yi's abilities. Navies are kinda meh in civ 4, but Gongsun Du's position in Qing (oddly), Northeastern China and Korea emphasize a lot of naval transport and sometimes even combat.
Zhai Rong (Tao Qian or Liu Yao or even both: Liu Yao is favorable since that faction has an in-game Buddhist focus): Zhai Rong is one of the most bizarre figures of the period. According to Rafe's tome, he was a Buddhist man (somewhat rare for the time) who joined Tao Qian. Under Tao Qian's service he embezzled supplies and used them to build a Buddhist temple with several thousand followers. Later he fled Tao Qian's lands, killed the Administrator Zhao Yu, and joined Liu Yao. He fought Sun Ce under Liu Yao's service, was defeated, and then ended up betraying Liu Yao. Briefly he took over some other land. Eventually he was killed after incurring defeats against Liu Yao. He'd probably work as a melee officer. Maybe could have some skills in siege since there aren't too many of those.
Xu Rong (Pass-West): Xu Rong is one of the most underrated officers of the period. He was essentially Dong Zhuo's best officer after Dong Zhuo came to power. According to Rafe's tome, he was responsible for defeating Cao Cao at Rongyang as well as inflicting a defeat upon Sun Jian. I believe this defeat was the same one where Zu Lang saved Sun Jian's life, but I'm not 100% on this. The problem with Xu Rong in the context of this mod is that he resisted Li Jue and was killed fighting Li Jue's party before the main scenarios of the game (he died in 192). Given that I can see how it might be difficult to incorporate him. However I have another suggestion as well.
Fan Chou (Pass-West): Fan Chou was a pretty significant general of Dong Zhuo's and Li Jue's. He was instrumental in beating back Ma Teng's invasion of Chang'an. However he fell out of favor with Li Jue and was executed. Sometimes Li Jue's faction is considered a triumvirate with Fan Chou being the third wheel aside from Li Jue and Guo Si. Both Fan Chou and Xu Rong I think would work best as dominantly melee officers with an archery sub-focus.
Some other information that may or may not be useful:
Gongsun Du called his state in Northeast China/Korea Ping (平

.
Yuan Shu called his kingdom Zhong. I don't have the hanzi for this though. Sometimes it is mistakenly called Cheng, apparently.