I always find it useful to examine the true nature of magic when figuring out effects. Combine that with a bit of physics, and neat ideas begin to form.
Generally, most magic either entropic, decaying, destroying, or reducing the energy of a system, or extropic, by bolstering, increasing, or enhancing the energy of a system.
At its heart, summoning magic is rooted in neither the entropic nor extropic. It is instead the power of translation and transformation, of taking that which already exists and moving it and giving it a different form, though not necessarily by making it any lesser or greater than it already was.
Applying this reasoning to skill creation and a necrotic theme, some unique ideas suggest themselves.
What you could do is have the Necromancer have 1-3 passive buffs. Good buffs. But he can activate a pet, losing that buff, essentially turning it into a pet of the same theme. For instance a thorns aura-type buff could turn into a tanky minion. When that minion is unsummoned, the Necromancer regains his buff.
You could have a dispel magic that summons minions when it dispels.
Don't forget the spectral aspects of necromancy, either. I always wanted to make an ability that would basically be a Shockwave that teleports you to the target location. Make the projectile/wave a ghostly or spectral form and bam- you have a ghost rush. I always imagined that power for more of a warrior type, but a spectral application is extremely appropriate as well.
For a spectral type summon, if you could find a model that is invisible except for weapons, it would create a unique and powerful visual effect.
I'll have more ideas, but I'm extremely tired.
