Since it would be an on-line app, whether on computer, iPhone, etc., it doesn't matter what cards you do or don't have, as you should have access to them all.
However, if I were the enterprising individual doing this, I would restrict cards by power levels and have players play against one another based on the minimum available overall power level in the game. That is to say that a low-moderate power card, a unique, would be, say, level 2, and a player would have to accrue so many points against other players before being able to gain access to new cards. Alternately, cards could be sorted by power level, again, by the programmer(s), and then either a level of cards, or the individual cards, could be "purchased" for use.
Anyway, when I say minimum available overall power level, I mean that, of the players currently active in the game setting, if you sign up as a fresh player but the next lowest player is, say, level 3, your account would automatically be bumped to level 3 so you could actually participate against other players. If the level 3 player(s) aren't playing often enough, anymore, you could then petition the GM to bump you up where other active players are.
