Psionicist/Necromancer
This combination has interesting possibilities. Many psionic disciplines (particularly psychometabolism) include abilities which affect the processes of healing, disease, and death - perfect powers for the necromancer. For instance, Tanith Lee's novel Kill the Dead explores the theme of a psychic ghost hunter whose mental powers enable him to sense and manipulate spiritual forces. Indeed, it is easy to imagine how dabbling with such psychic skills could lead to a wizard's career in the Art.
If the DM is interested in further exploring the theme of the necromantic psionicist, refer to DRAGONŽ #200, where "The Even More Complete Psionicist" details the Spiritualist Kit: a psionicist who specializes in communicating with the spirits of the dead. The Spiritualist could play a similar in role in a campaign to the Witch necromancer who serves primarily as a diviner. For more psionicist kits and further ideas to flesh out a dual-classed necromancer/psionicist, see also "Open Your Mind" in DRAGON #191.
Along with the requisite 17 Int, this dual-classed character would require a 15 Con (in addition to the usual 16 Wis). An NPC's level of experience as a psionicist may be quickly determined by rolling 2d4 (resulting in 2nd to 8th level). While the character would undoubtedly benefit from a psionicist's wide range of mental abilities (as detailed in the Complete Psionics Handbook), the added complexity could render the NPC difficult to role-play effectively. Those DM's wishing to integrate psionics with the necromancer in a more limited and manageable fashion may care to employ wild talents rather than full-fledged psionicists.