And for all the high level necromancy spells? What if wizards had no spells but gained fireball as class ability? This game has an existing system for these sort of things, it is pointless to invent a parallel system that does the same thing.
Honestly, I don't actually even care about the idea of necromancers as their own class. But my idea was, if you really want a class that emphasizes a specific school of magic--whether necromancy, illusion, or any of the others--the idea that the things that make that school interesting should be built into the class and not dependent on spells. Since let's face it, the typical player is going to take whatever spells they can get, no matter the school, unless the DM has houseruled it to make it harder for you to learn magic outside of your specialty. So there's a couple of possibilities here:
Full caster. The class spells are more limited in scope, not broad like the wizard. The necromancer gets necromancy spells plus a handful of other spells. Because there's no reason why a necromancer needs
fireball.
Half Caster. Spells up to 5th level. But all the really necromantic spells are built into the class. So a necromancer archetype built around manipulating life energy would get an innate ability to (perhaps) suck the life energy out of one creature (maybe even themselves), store it, and give it to someone else. This would mimic
cure wounds/cause wounds and even
heal/harm. A necromancer archetype built around undead would start being able to make undead early on--maybe even as soon as they take the archetype--and as they progress they can make better and better undead. So this sort of necromancer could make a ghoul at, say, 9th level where a standard wizard casting
create undead would have to wait to 11th level. And an archetype built around manipulating souls would get some innate abilities that mimic spells like
soul cage, but again with more options as they level up.
And you can also give them a warlock-style Mystic Arcanum so they can still take the less well-themed necromancy spells, like
eyebite, which is necromancy only in the sense that it's
spoooooky.