For the record, I'm looking at a character that goes for the traditional Eye of Newt approach, fire burn and cauldron bubble and all that. In terms of actual magic, he might be more or less incompetent, but, when brewing potions, he is peerless, and so is something a 'fixer'. You want something doing, you go to my guy (and pay him). He is a craftsman, one that will make magical artefacts, or use his potions as a sort of 'fuel' for big, wide-ranging magic with spells that affect the weather, or luck, or wider concepts. Note that this makes him a valuable ally for anybody that is not themselves a witch because he can perform magic on their behalf.

Hopefully, another player's magical family will have my character as a sort of 'mage-in-residence' where he would have official ties to the family and all the benefits and duties that that implies, but with slightly more independence and no blood connection.