From the inimitable and laudable mind of Contraserrene (see the previous post on the boneshaper fae for another of his contributions):
For an upcoming campaign I've designed a human magister who's replaced his staff with a slave- he keeps a dominated spryte leashed to his wrist with an ensorcelled chain. When he casts spells he's channeling his power through the spryte, who provides the verbal and somatic components.
Mechanically, he can cast his spells without worrying about somatic components (allowing him, along with a feat, to wear armor without a spell failure chance). He also gets an extra pair of dull-but-obedient eyes and ears to watch for stealthy attackers. He also gains additional weaknesses- knock out the spryte or break the control and the magister has problems. He's not doing it for power; he's doing it to show off what a badass he is.
I was thinking of generalizing it into a feat with a less malevolent twist, but I haven't worked out the mechanics for that. It would be closer to a familiar than anything else, I suppose.
Edit: Needless to say, once this magister goes public he'll become something of a priority for the faen community...