Necromancey isn't even OUT OF THE GATE YET and I'm already making a suggestion! THE GAUL!
That being said, I'm not sure what Toady has planned ((and he never disappoints)) but I've a suggestion anyway. The undead or simmilar unliving at a necromancers command are powered by the foul miasma given off from the dead, we already have miasma IG, why not make it the unlife that the evil dead sustain themselves on? Expiring once again not necessarily from sustaining blows, but from being otherwise deprived of the arcane mist?
*A visual aid*
It adds another aspect to Fortress mode in that you could fend off ((or at least significantly weaken)) the minions of some twisted corpse raiser with soothing inscense burning throughout your mountain stronghold. Likewise if you have the halls filled with noixious fumes as thick as the carrion strewn pit from whence this evil was spawned...good luck.
It also adds a bit of management to the Necromancer rather then infinate power or some out-played mana pool concept. A more powerful necromancer is a more prepared one! Nigh unconquerable in the near suffocating fog of his sanctum. More powerful Mancers could perhaps store up some copious quantity of it within a urn, reliquary or even their own bodies! A literal kiss of death at their disposal.
Need I go on? Using, amassing, combating or enhancing this simple concept no doubt surely abounds with creative possibility.
Instead, a tertiary thought in passing, that has been said in more then one other post. A Human skin cloak imbued with unlife that will make wrestling attempts against your foes when worn, or kill outright anyone foolish enough to put it on.