Also the abyssal units and cronebound.
I think Twilight-Kin are a good example of how to get a theme list right too.
They let you do something you couldn’t with allies (take units from 3 lists) and do something different with those units (long range drain life).
Butchers in Twilight-Kin play differently to in Night-stalkers (right?).
Another example is the Brotherhood.
Order of the Green lady offer a combination of knights and elementals that you couldn’t quite do otherwise (sacred water synergies could be better though).
Brothermark on the other hand have cool unique units, but you can do a very similar list with KoM or Basilea.
While I would like some fluff, I’m not sure this is the right approach.
You can take a list and theme it through unit choice and modeling/painting style to be unique from another army from the same faction. KoW has the flexibility to make an army your own.
Theme lists need offer a unique experience.
The mercenary ogres plus humans list @MithrilCoyote suggested sounds interesting. Could have a variation of Indomitable Will that’s themed to mercenaries.