I like the cards idea, I used to play alittle MTG...if you want to change them, I would suggest changing the "lands" into "boobs"
Then have different articles of clothing as "weapons" and have the boob stars as some of your creatures.
And for your instants you could put all the standard stuff in there, Instant BE, Gradual BE, Butt growth, Giantess etc.
And rather then have the summons fight you should say that they have energy until "they burst"
And make multiple sorceerss model's for the user and everytime they get a "BE point" have a drawing (or CG) of that model's breast growth.
wow, that was a spark of imagination.
If you've got any questions about what I just said, let me know. email is
[email protected]