I've just been scanning around looking for stuf on halo 2 and theres a multiplayer map that is unlocked once you complete the game
theres a bit of faffing to do too, but it looks like a cool small map ideal for a bit of "license to..."
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regular readers of xbox-scene may have already seen this, but there is a halo2 modding group being established that will end up making new maps and guns and things
there is also this little mod that maybe of some interest to tart
blatant