Actually, I just thought up the following as a way to unify a bunch of pets under one roof. It's more balanced, and basically makes the druids animal companion equal to the wizards familiar. I don't have any of the actual crunch down yet, but it's coming along.
All familiars fall into a few different categories.
The first of these is type, which dictates in which ways the familiar advances. This categories are usually magical beast or construct for arcane casters (dragon and undead are also available via feats or ACF), while druids and the like usually have animals or magical beasts, but swarms of vermin, plants, fey, and later monstrous humanoids and elementals are all available to druids. Clerics have the chance to obtain an outsider familiar. Artificers can choose a creation of theirs to be a familiar.
The second of these is subtype. A subtype affects minor parts of the “type” progression. Not all familiars have a subtype. Some familiars are swarms; this is the only major ability that a subtype will net you in terms of a familiar.
The third is path. This determines the focus of the familiar. This works like class of a character. A familiar effectively gains levels in a path like you do in a class. Multiclassing (or multipathing) is possible. For example, an artificer might want his familiar to craft better but also wants him to be able quickly use infusions. Many classes restrict what paths their familiar can take. This replaces normal advancement.
The fourth is class. A class adds minor bonuses to his familiar, as listed below.
