You should add weapon drop pods and covenant computers and weapon pods things.

Also, I was thinking about this. This doesn't really make sense if it takes place in the same lake as the one in H2. What might be a better idea is if the Halo 2 location is expanded on, taking place in an adjacent lake. I think at the beginning of The Sacred Icon, you can see more than one lake, but I'm not entirely sure. Even so, a little extra could be injected.

With the surrounding land, just make it look better. Add trees and bushes, much what you see at the beginning of the level Delta Halo.

Hell, what might even be more awesome is to have the covenant cruiser as an actual model (low poly, though) way in the distance, but not just a silhouette since it could take closer to it. Most could stay the same except for the wall and cruiser positions. Hell, you could even animate the covenant fleet slipspaces in the air. Too bad we can't have animations in the main menu though...