i have a good one (if you want to make a banner for it go ahead)
it would be "go to hell" where you have to go to the nether to get it.
i cant believe notch didnt add that one in
and a different one i have is
traitor where you would have to have a skeleton kill a creeper
How To Get: Have A Skeleton Kill A Creeper.
I Gave It Different Name. "traitor" doesnt make sense because they have always hated each other.