This is a great idea, however wouldn't this remove the urge to collect cards and make a deck out of them?
Why spent so much effort getting a 120 deck if you could have a random one each 2 weeks? Ofcourse you won't be able to pick them yourself but I can see people not wanting to bother create their own 120 deck if they can get a 120.
Therefore I would suggest making it like a 100 deck so you'll benefit more for making your own 120 deck.
Nontheless a great idea
Edit: Or you have to rent them thats also an option, this way you would save BFP by making your own deck in the long run