You sort of have it backwards when it comes to CE's story. The idea of exploring the ring and the flood being this big twist that would turn the game upside down was what they started with, and everything else was built around those 2 ideas. Which in the end worked great as it introduces you to new things in each of the 7 unique missions, and then shows you how all these new sandbox elements that the story spawns can effect the older levels you visited earlier in the game.
But the whole thing with H2 was that the entire game is more driven by Staten's story, with maps and the sandbox being made to support his narrative, rather than the narrative being constructed to do the gameplay they wanted. The brutes are a good example of this, having been initially created just as a new enemy to have as part of the Covenant and then repurposed to be the new main bad guy, despite them being completely ill suited for the role. All of the Arbiter missions suffer from this too. Does anyone really like the Halo missions where you are not fighting the Covenant? Even H4 only does that in it's last mission.
Basically, H1 has a plot twist because they wanted to add a crazy new sandbox element. On the other hand, H2 focuses on it's plot twist at the expense of diversity in the sandbox.
With SPV3, our extended sections just tried to build off the existing things that made those maps unique. Some of them were more successful than others. The project was more so an example of how I'd like to see the sandbox have been expanded in Halo 2 and subsequent titles. H1 works great with it's 3 enemy weapons (excluding sword and fuel rod gun) and 4 species, but once the player learns the ins and outs of the mechanics, there is little more to learn or be challenged by. In H1, it's really quite amazing the gameplay still holds up, whether it be to a due to the ai behaviors, thoughtful weapon sandbox, missions all being unique enough to offer something different than the others.
What we really set out to do was expand on the things that worked great and we loved in H1, but add new content that would give people even more mechanics to learn and challenges to adapt to. The biggest problem with that is there are so many new layers of complexity when it comes to guns to use, enemies to fight that it can be overwhelming for newer players. In fact, we did have a fair amount of complaints about the difficulty from people when we released, but many of those people after sinking in 10-20 hours of it then "got it" and seem to agree with our decisions (other than a few but that's not really a discussion for here). We probably added a few to many things in the end, but the fact you can run though AOTCR with the choice of 3 different hog types, a grizzly tank, a shadow, ghost, wraith, aa wraith or banshee if you chose to gives you the chance to have a totally different experience interacting with everything in the level each time is pretty cool in my book.
