I would probably allow a number of different feats in exchange, possibly two feats. Another good option, is the Staff option that was located in a relatively recent Dragon Magazine. It exchanges familiars for a 'signature' staff that can produce special effects, effectively a Staff-Familiar (and gains additional hardness/hp as the caster increases in level). It even forms the basis for an additional feat tree based on the staff.
As to other feats: Alertness, Eschew, brew potion, Weapon Focus (touch, rays), Spell Focus, Spell Penetration, and a number of Forgotten Realms types (Tatoo focus, Shadow Weave, etc) because I tend to DM in the Realms.