Somehow, I managed to lay hands on a copy of GURPS Thaumatology: Magical Styles before I actually had Thaumatology proper. It left me scratching my head a bit, but it also fired my imagination with ways to use the default magic system in new ways that made more sense to me than the sometimes arbitrary system of prerequisites that come standard. I thought that might be the best bet for building specialist wizards, like someone who approaches things from a shamanistic or animistic perspective.
There are a number of useful spells for the spirit-minded mage, but you might not ever think to take them, buried as they are deep in the Necromancy college, a decision that I suppose could make sense depending on the theology of your setting, but it does them a disservice in a game where “spirits” aren’t synonymous with “souls” for example. The biggest issue is that they’re all pinned behind the spell Death Vision which doesn’t necessarily suit a character who doesn’t have evil in mind.
So I set out to begin creating a Spiritualist style using the rules presented in Magical Styles, and begin by mining the Necromancy spell list for a core of tools:
Sense Spirit – to detect their presence
Summon Spirit – to interact with the recently departed, or even the long dead for the sufficiently skilled
Animation – summon a spirit into a manufactured vessel
Materialize – to force a spirit to become visible
Solidify – to force a spirit to become tangible
Affect Spirits – allow the mundane target (person or object) to interact with spirits
Skull Spirit – for those of an offensive bent, a spectral assassin
Turn Spirit – to force a spirit to flee from the location
Astral Vision – to see the spirit world
Command Spirit (type) – control or dominate spirits
Entrap Spirit – trap a spirit in an object or location
Repel Spirits – protect an area from spirits
Bind Spirit (type) – like Command, but permanent
Astral Block – more effective protection from spirits for an area, with other in-built protections
There are certainly other spells that this sort of wizard might want, but this is a robust and useful tool-set with a focus on the astral plane. From here, we can work out some alternative prerequisites, skills that suit the feel of the style, and add in some perks that suit as well.