ToME: the Tales of Maj'Eyal

Some Necromancers do actually know Soul Rot in the game (Blighted Ruins), so some already are technically a Vim class.

Thing is though, would new Necro be a Dark and Blight damage class? Would it keep Cold around for Wisps and such? What locked tree replaces Ice?

I'm going to agree on 'souls as a cap'. What I think would be good is:
Every time you make a minion, it takes a soul. When a minion dies, you get the soul back x turns later - the necromancer's hold on the soul does not end with the undead minion's destruction.

Create minion should then create 1 minion per cast; you don't necessarily want to use all your souls on minions. You then also have room to specify the minion you want to make. Essence of the Dead talent, on the other hand, would 'shotgun' random minions at the enemy, consuming all or almost all of your souls.

I also think vampires and lichs should not be 'simple' minions, that can be popped out with the same limitations as skeletons and ghouls. Lichs especially should be a big deal; you have this huge quest as part of the class to become a lich, and you can randomly pop some out as cannon fodder?

I'm thinking a talent that gives you one big pet - either a vampire, a lich or maybe a bone golem, picked when you take the talent. May or may not be golem-like with talents. Alternatively, they are separate talents in the same tree, or each have their own (presumably locked) trees.

Could be interesting to have a 'lich' tree, where the third ability is the lich transformation and the fourth creates a lich minion, and then have the same kind of tree for vampires.