In the campaign of Halo 3, in the first level there is a bridge towards the end. The bridge has a tower on side, and is covered with brutes. Also, there are tunnels running under the bridge. During my run through of the campaign, I noticed what a killer map this would make. Anyone want to help me? (Oh yeah, don't steal my Idea. I will hunt you down and force you, against your will, to listen to every Britney Spears CD ever recorded.)
Campaign multiplayer levels would be sweet, but it's hard to actually remind somebody of the level because of the setting of foundry. But you should definitely create it, i'll keep my eyes open for it.