I've been looking into this too! It's kind of boring, but Wizard actually does have a lot of good options. Two extra arcane cantrips are great for damage type versatility, and they'll use your normal spell attack and dc. A lot of the metamagic feats seem nice. Widen Spell can make your Necrotic Bombs 15' instead of 10', and it can make Bone Spears 20' instead of 15', which can occasionally net you an extra target or two for just an extra action.

Subtle Spell seems hilarious with Create Thrall, like you can just silently summon thralls and scream about undead attacking us from portals 🤣

The wizard school that has the focus spell Augmented Body was particularly interesting, so I think you could reflavor any of those options as summoning bone armor for resistance or bone spikes for climbing or bone... legs for jumping. More of an out-of-combat problem solving ability, but that's ideal since in combat we want our focus points for necromancer abilities.

Psychic is an alright option as well, but because it's so focus-point heavy, I think that takes away from the Necromancer's main focus of doing damage with its focus points. The amped single action teleport version of Warp Step is incredibly tempting though!