Flavor wise, this doesn't make sense - a balanced ecosystem balances it out between one or more creature types. I'd also like to see you pick and choose a creature type, and then maybe you can bump it to 1G a la Shared Triumph. Or what about something like this, where you choose a creature type to have the cost increased by 1, and one creature type to have the requirements decreased by one.
I almost went that route...making you pick a creature. Also the names I put on my cards are just placeholders mostly. Obvious it won't make sense at all if i change it to pick one creature type over any other. It would probably have to be called something like "Dominant Species" or "Gaea's Favored".
Edit: I could make this a pack hoser. Like "at the beginning of your upkeep if a player has more than four creatures in play that share a creature type, that player sacrifices one of those creatures." Would that be a better choice and make the other idea a different card all together?
