Hmm.. I would be nice to have some kind of wildcards craftable.
Lets say you can trade 5 cards into a token of the same rarity. These tokens can be exchanged into other cards of the same rarity. This would help the players getting hands on cards they really want for their decks, while shredding alot of others.
The obvious problem would be the price differences (who wouldn't like to buy 5x Curse Orb with ~40 bfp each in order to trade them for Enlightenment?). In order to prevent (or at least lessen) this issue, the cards could require a different amount of tokens, if you want to get them. So a Enlightenment would cost lets say 30 tokens while Curse Orb only costs 1.
In order to determine how many tokens a given card consumes, we could use the long-term average on AH prices (at least one month average or even better median in order to lessen the problem of manipulation). This could be done Static - a wildcard of a given rarity is worth x bfp and the cost for a card is AH-price / wildcard value (rounded down) or dynamic (lowest 20% of cards cost 1 token, 21-30% 3 token, etc).
Sure, the devs could also determine fixed prices for each card, but I think that would cause a lot of extra work and the system doesn't reflect price canges in AH. For example releasing a new card causes a existing one to go up from 50 bfp to 500 bfp because of a new combo. A dynamic approach like described above would reflect such changes, even with some (debatable) delay.
The big difference between my token idea and the already mentioned "x for one random" idea is the lack of RNG. I know some of you really like gambling, because it's exiting/fun. but on the other hand there are also some players (like me) which prefere a more predictable result.
/edit: after the reply from Asraiel, I edited the wording in order to make the idea more clear.