Dude, that's pretty harsh to say over an idea. I get that you're entitled to your own opinion, but someone expressed the desire to make something new, something that's practically non existent in this game only to have you shoot it down because it's not a blasted breedable! Has it occured to you that because you're not the target demographic doesn't mean that no one is? If you don't want a TCG in SL, fine, don't buy it, but don't stoop to insulting someone's idea because it's not something you're interested in. That's just shameful.
I for one would hate to see more breedables and would greet a concept like this one with open arms, simply because it's a new concept. Breedables were cool to me when they first became popular, but now it's a concept being bled out by anyone with a scripter and a sculpter.
As for this idea, like any game, both physical and digital, it's best to start with pen and paper. Figure out the theme of your game, what should the cards represent, what are the rules for each type of card, how would a game play out. test this idea with folks you know and see if they find it interesting, then all you need to do form there is determine how to implement it in SL in a way that would be appealing for those who do buy it. Implementation in SL will be the trickiest part as the guy I replied to said(Sorry, your avatar name blends with the picture too well), it has never been done before; so you and whomever works with you would have to figure out HOW to make it work. That means distribution, marketting, game flow, valuing the cards, promotion, and every other little bit of detail that would be invested in it.
The easiest part of every project is the idea, but once you start turning that wheel, you'll have to be prepared to face the challenges behind creating it! So If you do undertake this issue(and don't let ignorant comments like the one above discourage you), you can probably be the pioneer of SL TCG. That in itself, sounds pretty cool!