mob area rearrangment

well, i had this idea that i feel will help the realism of the game. notch could move the zombie and skeleton spawning areas to nether so that only realistic creatures like spiders, and possibly things like bears and alligators could spawn in the overworld, and the more nonsensical mobs would be confined to the nether. the only acception would be the creeper. i dont want any fucking creepers teaming up with ghasts.