Here is one from the Madden playbook (bad pun intended).

Vitrual collections with virtual card rewards. If you have ever seen Madden Ultimate Team, you know what I am talking about. Basic idea is you submit cards to a "collection" and are unable to use that card again. When you complete the collection you would receive a reward in the form of Gold, Crystals and/or Cards (new, regular, combo, limited edition, etc.).

Good for all:
Players - Get the fun of trying to complete collections and receiving rewards
SE - players try to collect all of the cards for a collection so they can receive the rewards (They have to play/buy more)
New Players - Opens the game up to "collectors" who still want to collect.
Atmosphere - A fun way to roll out new cards or even combos for a given class.
Marketing - Gives SE the ability to run limited time promotions through the collections.

A trading system would be necessary for this to truly work so players could trade extras to complete collections. It might also be wise to offer different types of booster packs. Either different size packs/boxes or ones that can focus on certain classes/types of cards. Would really be cool if you expand into different editions with unique strengths/weaknesses.

Just some mid afternoon ramblings. I did a search for collections but came up with nothing. Sorry if this has already been suggested.