Okay, sorry, don't want to hijack your army idea. But you pushed my button, guess which one that is? Also, no critique meant in this post, just inspiration, hope it comes across as this as well:
Okay, Fun idea (but a minefield)! Seeing as you've actually researched some of the real names for inspiration (bokor et al) why not go the whole way ?
Vodou has several pantheons (Petwo fiery and aggressive, rada family and healing etc etc), and instead of using kongo as your death faction, which is a little off seeing as kongo is a semi peaceful and colourful pantheon in vodou. Why not the call them the Gede alternatively spelled Guede, the proper death faction in vodou?
They could be led by the family of Barons that lead the real one. Baron Samedi (Baron saturday) you know the guy in old fashioned mortuary suit with tophat big boss of the dead. Also Maman Brigitte (Mother Bridget) and Baron La Croix (Baron the Cross), Baron Cimitiére (Baron Graveyard) the protector of the graveyards and the only guede/kongo/petro crossover Baron Kriminel the feisty fiery devil of the gang (and also maybe the most scary of them all) -name says it all.
Also the gede, though they can be scary sometimes, are not evil far from it, which is nice from a fluffy point of view. I tire of the evil/good dichotomy of some of the races in warhammer. Every faction should have some kind of entry point for the player to be able to identify with it. Baron Samedi both takes and gives being the key holder to death, but he's also a powerful healer and loves especially kids, protects them like all the gede do. This opens up for human followers further than the houngan/bokor type. Maybe the children in the town were infected by some sickness and everyone started offering to the Baron for help? He helped them. In return its his town. Everyones happy. Loads of cult members.
And why not have possessed? Those are your troops with inhuman powers and no thought of themselves. And drumbeaters, kind of like the ironjawz war chanters to keep the possessions going and giving +1 to hit and an extra save or something.
This would definitely be a magic heavy list with all those vodou priests: houngan and mambo (male and female vodou priest working with traditional vodou -usually) and bokors working with the left hand dark magic curses, zombies and all, as well as the gangan - traditional leaf healer.
In haitian vodou they also have loupgarou -werewolves and many other weres- actually, weredogs werecrocodiles, werecows you name it, - so why not go for these as well. Maybe the cows would be a bit off! They'd definitely be under the bokors. Theres the secret societies also, which mete out justice, when this is needed. Groups of magicians, leaders of the communities, meeting in the dead of night to hold mock trials, sometimes ending in punishment of taking away the perpetrators soul (zombification) and the like. Again, they are not bad, they only meet when judgement is needed -and everyone knows the sigmarines should be judged by others than the inhuman god Sigmar! maybe styled somewhat like the 40k inquisition? Stealthy judgement is the vibe I get of those, knowing next to nothing of 40k.
Of course, you already have inspiration for the colors as the gede and the Barons primary colors are purple and black, with the followers of gede having flour blown onto their faces to emulate the white skin of the dead, somewhat like orruk warpaint. They also wear hats and sunglasses with only one eye having glass in it (as they have an eye in both the worlds of the living and the dead) and smoke copious amounts of cigarettes. Often two-three-four-five cigarettes in their mouths at the same time! Bad ass. Top hats! Baron Kriminels colors are RED.
Ah, sorry, for the rant. Just some ideas for your fantasy faction. I'd probably go for something else myself. Thats a loaded subject!
T