One plot idea could be Dark Naruto using necromancy to bring back past villains to form an army against the Hidden Villages. He might do this to seek revenge for the way he was treated. Another could be that he uses necromancy to communicate with the spirits of his ancestors to gain powerful jutsu knowledge. For example, he could bring back the First Hokage to learn about his wood - style jutsu secrets.
His powers could start small, like being able to sense the lingering spirits in a graveyard. Then, as he experiments and learns more about necromancy, he might be able to briefly summon a weak undead creature. With time and practice, he could control larger groups of undead, and eventually, be able to raise powerful dead ninjas with their full skills intact.
For the dark witch, you could start by giving her a backstory. Maybe she was once a good witch but something happened that turned her dark, like the loss of a loved one. Her powers could be related to shadow magic and curses. For the necromancer, describe his connection to the dead. He might have been drawn to necromancy because he could communicate with a deceased family member. His appearance could be pale and his eyes have a faint glow.
One unique element is the new power set for Naruto. Instead of just his regular ninjutsu, he has necromancy. This allows him to interact with the dead in ways that are not possible in the original story. For example, he can raise the dead for information or to fight for him.
A 'naruto is a necromancer fanfic' can be interesting due to the contrast it creates. Naruto is known for his positive, life - affirming values in the original series. As a necromancer, he has to deal with the darker aspects of death and the afterlife. This internal struggle can create a really engaging story. Also, he could use necromancy to bring back fallen comrades, which would change the whole dynamic of his relationships. It might also introduce new enemies who oppose his new - found power.
One plot idea could be that the dark witch and the necromancer start off as enemies. The necromancer is raising an army of the undead that is threatening the witch's forest realm. But as they clash, they discover a common enemy that is manipulating them both. So, they have to put aside their differences and work together.
The most interesting feature is the unique power set. The necromancer bloodline allows for abilities like raising the dead or communicating with spirits, which is very different from the normal Naruto jutsu.
I'm not sure of a specific 'naruto is a necromancer fanfic' off the top of my head. But you can try searching on fanfic platforms like FanFiction.net or Archive of Our Own. There might be some interesting works there.
In a 'Necromancer Harry in 40k fanfic', the setting could play a huge role. The 40k universe is filled with various planets, each with its own unique environment and challenges. Harry could find himself on a death world, where his necromantic powers are the only thing that can keep him alive against the native predators. Or he could be on a planet ruled by a tyrannical warlord, and his powers make him a target. The contrast between the magical world of Harry Potter and the gritty, war - torn 40k universe would create a lot of opportunities for exciting storylines.
Dumbledore, if he were still alive, would be very cautious. He would try to guide Harry towards using his powers for good, or at least in a way that doesn't disrupt the balance of the wizarding world. The other Hogwarts professors would likely be split. Some would be horrified and want to expel Harry, while others would see it as an opportunity to study a rare form of magic and its implications.