There should be a stand alone, with hexen's 3 characters, and all the hexen enemies. The expansion should add all the textures, enemies and the heretic guy into Scattered evil. The heretic guy can hold more weapons than the other classes and does not have any super 3 piece weapon. And can also use tomes and shadowsphere but cannot use hexen's artifacts. Like in a RPG, some classes can use some things but others can't. I'm sure you did this before such as cleric cannot use bracers.
In scattered evil, the end boss will be something
In the expansion, the end boss should be D'sparil, then you fight korax, then the chaos sphere gives them life and now D'sparil hops onto Korax so D'sparil is riding Korax. They get resurrected somehow and now you have to fight them both. You have to deal with the traps as well as the Disciples.
To kill the D'sparil and Korax thing, you have to destroy the chaos sphere first. And then a doom engine version of Eidolon, seeing that the sphere is about to be destroyed comes in and steals the sphere and D'sparil riding Korax becomes vulnerable. Then you kill them.
The story does not have to connect to Hexen II. The expansion can be before Hexen II is. And somehow while changing from the ParthorisCronos world to Thyrion, the sphere regenerates itself.