Without wanting to sound like Mr. Negativepants, i've got a question.
@TO and the other long-time players. Taking TO's first post into regard. In little time you've accumulated quite some ideas here, but how long are you willing to wait for them to be realized? Another 30,000 games?
In the past 2.5 years we've got 3-4 cards per quantum. At the current rate you'd have to wait app. another 4 years until you reach 20/q (not baring codechanges for generally or individually reduced cardlimit nor the bonecrushing work of balancing cards afterwards).
From the view of the amount of work one would have with reducing the card limit, the step down from 6 to 4 would be more preferable, than the fluid 6,5,4 method.
A player may consider a fluid approach more preferable, because it seems to be less of a hard line cut into the system, but a strong cut would reduce the amount of work and basically almost reset the entire deck-building approach, which should pose a little bit of a challange even to long-time players, which is what you want, right?