I personally would say there should be a minimum of X card slots that are not filled with a specific card and that those open slots should draw from a *combined* pool of at least Y cards.
I'd set X=4 and Y=30, that means you can have at most 6 fixed slots and need at least 2 different expansions among the open slots, in which case at least one must be a 'big' one (Say 6 fixed cards, 3 slots for Intrigue and one Promo card slot). If you leave one card slot totally open (no required expansion), that should fullfill the Y conditions usually alone, unless someone is really only familiar with base cards.
That should give people lots of options for their familiar cards or banned cards, without being able to overly game the system. Of course, it is debatale if e.g. only playing kingdoms that contain 6 fixed cards (say always a viable draw engine, or always a Goons engine, etc.) should be the basis of someone's rating.