Along with a card we build our deck around, why not also a card (from the same element, has to be a different card) that we can't use at all?
Um... if the card selected that way is a key card for the element it couple be quite a handicap. If I drew a Hope card but photon was prohibited for instance I think I would just skip arena for the day. I don't think FFQ would be worth trying in Arena. I think it might just encourage more rainbow decks.