The parent poster is probably correct in assuming that each level will be a different Halo... unless of course the franchise is going to be X games long with no change in concept...
I'm going to guess that each ring will have its own climate, foilage, sky, terrain textures, and everything else that makes a environment... whether they all be part of Halo 3 (if that is indeed what it's going to be called) or seperate games.
We're also probably going to see some new alien life forms on the rings. We have already seen the brutes introduced... and with the way the 2nd Halo ended, I wouldn't be suprised if we started to see a heirarchy of brute soldiers as a "3rd race."
As for the weapons... they're all about balance. If there are a huge variety of weapons, but they don't play well, we'll have to scrap some. It's just the way of the world. It's the same reason some of the super-cool stuff was taken out before the game went gold.
We all would have loved to see all of the vehicles that were shown in concept art and concept renders... but they changed the gameplay, and for worse.
The next Halo is going to be awesome guys, just wait.
(And by wait, I mean "Duke Nukem Forever" style wait.)