I never done such character, but on top of the obvious one you mentionned, I would consider an Ex Miscellanea with several supernatural abilities - the Unstructured Caster flaw could be part of the tradition, they have so many supernatural abilities that it is virtually impossible to open their Gift without damaging it or loosing some abilities (and obviously, the tradition value more their abilities than hermetic magic), and since they are so bad at casting spells, they would spent anyway most of their XP in their abilities instead of their Arts.
It is a flaw that thematically could go well for any recent group of hedge wizards integrated in the Order - possibly first or second generation only - with most of their hedge magic not tuned yet to hermetic magic.
For an older mage, it could be the result of a Twiligth episode gone horribly wrong, leading him to seek a life without magic casting. Think of Guernicus who forfeit magic in the later part of his life (according to "official" history, to live amongst monks, but maybe it was only because he could not use his magic to defend himself), or Mercere who had his gift so badly damaged that he could not cast anymore spell (again according to the official history).
If the mage is Unstructured caster from apprenticeship, he would obviously seek a path where casting is not required: labwork, theory of magic and Arts (including research from virtus to go beyond any books and tractatus he can get), in fact, definetily somebody who could spend more time studying mysteries to get power outside of mere spell casting. It could be one of these magus looking to become a Fae or a Daimon to finally be able to perform magic - not to achieve immortality, just to be able to enjoy magic casting. I have not look if achieving such results is possible for relatively young magus (let say 40 years out of apprenticeship).
Possibly a Criamon seeking through Twilight experience to overcome his flaw.