Problem is if Bungie gave us the tools to create content then nobody would buy their DLC's because I fully believe that (and I know I usually tease them) the little kids on these forums and playing this game probably could create far more interesting content than Bungie could ever possibly envision.
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From what I've seen in halo 3 and reach forge the community hive mind can make hundreds of maps using the same set of materials that look and play amazingly enough that Bungie would take the most exceptional ones and actually add them into the regular playlists.
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That's how griffball was made, so it makes sense