Make the northern state of Yan unite Warring States Era China.
The most important thing, I believe, is
a military reform similar to Zhao's.
Yan overran much of Qi once, but failed in occupying the country. I don't remember if the reason was guerilla warfare or just incompetence, but the Yan were pushed back. On the other hand, if they remained in control of Qi or annexed it, this could give them a large boost towards making them unify the country.
Of course, Qi didn't unify China, so I can see why conquering Qi might not help Yan.
It was agreed that
Yue_Yi was the one responsible for Yan's early victory, the removal of him by the suspicious King Hui of Yan (reign 279-271BC) was the cause for Qi's successful counter attack.
Had Yue Yi not been removed, he would have been able to negotiate an extremely favorable peace treaty with Qi (total conquest would be hard since Yan's allies withdrew after Yan occupied
Qi's capital, perhaps to limit Yan's power).
IOTL, Qi victory resulted in the weakening of both Qi and Yan, which paved the way for Qin's rise. In order to create a new superpower, Yan must either annex Qi entirely, or have significant gains in the north (which later became Korea or Manchuria). Yan tried the later IOTL, but not to the scale which could compensate for her geographical weakness of having no easily defensible heartland.
That would be the start of a Yan Hegemony.
Idunno, they seem like the most difficult ones to do this with (Chu and Qi were the two other big contenders) because of their position as a buffer against barbarians and their mountainous border with their two enemies Zhao and Qi. I'm challenging the reader to make the Yan defy their unfavourable position and unite China like OTL Qin did.
OTL military powers like Chu, Qin and Zhao thrived on expansions into barbarian territories because of their geographical locations on the periphery (unlike the earlier powers like Zheng and Song who later suffered from their location at the center because they had nowhere to expand into).
Yan's expansions could be northwards. Since "Yan State knife money has been found in Beijing, Tianjin, Hebei, Shanxi, Shandong, Henan, Inner Mongolia, Liaoning, Jilin, Shaanxi, South Korea, Kyūshū and Naha", it was plausible that they conquer some of their trade partners and build up new agricultural bases like Qin and Chu did.
So the steps are:
1) Military Reform to create a steppe-style cavalry etc.
2) Victory over Qi to eliminate a potential backstabber
3) Northward Expansion and Consolidation of new power bases (like Qin did to the western barbarians and Sichuan.)
While doing steps 1-3, keep on engaging in battles in the south to keep the army capable.
4) Final Southward Expansion and Unification.