Hello There!
I would say that Necromancer could have more of a support/crowd control role. DPS would be kind of expected as well and Tank would be interesting I think. I would say Necromancer would be more versatile than simply one concentrated role. For example
Support/crowd control: Necromancer would probably do necrotic damage, decay, and life steal. So as a support role, they could debuff enemies by slowing them down with decay-like spells or stunning them with fear-like effects. This could give main DPS characters and Tank a bit of advantage and a "breathing space". Of course, life-steal spells would make sense. They could heal themselves with life steal or their companions which would make them kind of secondary support healers.
Debuffs would also be good as crowd control that could help the caster to get away from danger. Of course, summoned creatures would be the main asset. Skeletons, ghouls or similar creatures would spread the threat level from caster and allies to the summoned creatures. This would be good for both crowd control and damage depending on the summoned creatures.
DPS: Well, I would say more or less all of the above. I guess that with classic ranged DPS spellcaster type, there can not be mistake. Of course, it would be fun to have necromancer as melee DPS, concentrated on poison and/or curse weapon buffs, with an occasional short-ranged spell.
Tank: This would be interesting IMO as Necromancer would be expected to be a squishy caster. Some protection spells and debuffs would probably be useful. But the most interesting part would probably (again) be using summoned creatures to throw at the enemy. Zombies, skeletons, ghouls, it could be quite imaginative. Summoned creatures would be the main tool actually for a Necromancer tank as it would require for the character to be constantly active. Summoned creatures do not have to be super strong nor to have too much HP, just enough to generate enough threat for the enemy to be distracted and be "alive" long enough for another undead bunch to be thrown at the enemy. I guess it would be hard to balance out and it does look a lot like a support/crowd control explanation, it is just a part of the idea for tank really, but I would avoid the classic approach for a tank with a lot of armor, HP, and buffs/protection spells on character themselves.
Still, Support/crowd control would probably be the best approach for Necromancer in a team play, at least in my opinion.
Sorry for the long post.
All the best!
MX!