Since a lot of the concern here is over how the card can be overpowered if a strong Patron card is selected, why not just make this a paired set of new Kingdom cards (Artist's Guild/Patron) that are always added to the Supply together? Then tailor the effect of the Patron card so that you might want a few but not loads. That way, accumulating them is an actual cost. Something like a Cellar effect with a boost for discarding Victory cards would be synergistic but not overpowering, perhaps. A set pair of cards with both in the Supply is a novel feature in itself, so they don't actually need to do anything else particularly innovative.