I'd have nothing against this is you encourage people to do something of value, for instance creating a card with a new (useful) mechanic, designing a competition or a PvP event, coming up with tournament ideas or applying for currently open staff positions.
Posting in a forum game is something you do for your own amusement and doesn't benefit the community. Inventing/introducing and hosting a forum game would benefit the community, on the other hand.
Something like an achievement/medal program has been discussed before. This could simply become a "challenge" thread of real value, but you need to change the OP drastically to up the ante. Encouraging low content posts and rep spam is tantamount to spamming, yourself, and will result in a minor penalty.