For one of my campaign concepts, Familiars would be necessary for any mage; instead of a companion or servant, the familiar would be, essentially, the wizard's spellbook.
A familiar is the embodiment of an immortal otherworldly being, who can posess many forms, but only certain creatures/things can be mystically imbued with the Familiar's spirit. (cats, bats, rats, owls). The familiar serves as a (perhaps temporary) conduit to the otherworld, and without it, the wizard has no access to magic.
if the familiar dies, the spirit is thrown off, and must find a new host, and returns anywhere from a week to a day later, to continue to tutor the wizard.
There would have to be rules for when a wizard needs a familiar's help to cast the spell, or gather energy. perhaps:
-the wizard's highest level spells must be cast while the wizard and familiar are able to speak to one another (must be directly coached)
-the three highest levels must be cast with the familiar within close range, so the familiar can serve as a magical conduit.
-all other spells the wizard can channel and cast without the help of his familiar.
A familiar who dies is ejected into the otherworld, and must return to the real world and find a new acceptable host; only the familiar can decipher an appropriate host from an inappropriate host.
While the familiar is linked with the wizard, the familiar can leave at any time, and begin tutoring another wizard. Thus, the wizard must treat the familiar with respect, or risk losing his familiar forever. Later, another familiar could choose the wizard as his acolyte, but, unfortunately, the familiar finds the wizard, not the other way around. A wizard can continue his studies (and gain experience) without a familiar as normal. A familiar who leaves brings all his knowlege of his highest spell level with him. He does not have the power to channel the magical energy of the 2 levels below that alone.
As the wizard gains levels, and the link between wizard and familiar grows, the familiar can adopt stronger forms, moving up from bats, lizards, owls and snakes and up to smaller magical creatures, eventually leading up to small dragons, and the ability to shapechange, perhaps even into human form.
Most familiars have other concerns, and spend at least 8 hours a day away from the wizard.