About the level progression, this made me come an idea.
What if the beach is act 2?
I'll explain. I don't see a beach to be a whole level, however putting it only at the start, like S3&K, may leave some potential out here.
Where would you place Green Hill Zone? What does it look like? I always wondered what GHZ truly was: you run on a long grass field with palms and flowers here and there. You suspect you're on an island. But the background is not, and if you take a look at the various 3D redintion it had you understand that GHZ is actually more similar to a green highway suspended (not really. coming out fits more) over the sea. And this is where I got this idea.
I recovered these 3 mock-ups from past posts to better explain my point:
1) First, the level starts in what looks like a "green highway" "suspended" above the sea. The first act could be a reminescent of GHZ and similar clichès. However, there could be some differences (some parts would be at the level of the sea, putting some water in, while GHZ was all above). The main concept is that this highway goes straigh to the main island. The level progression goes like this:
2) You start to see the main island in the background. Then the green highway fuses with the shoreline and the beach area starts. The beach keeps going for a little, while the "sea and island" background turns into a more tropical island background. Then:
3) The beach ends, and green highway reappears (or something with a similar pattern). But this time, it's on the main island, and it has a more tropical appearance. The backgroung is not really junglish, but it is more green, with waterfalls and trees.
Classic and new combined in one zone (and it fits with the surreal feels the zones always had).
This could give a good sense of progression. The beach will be in the zone, but the zone won't be made all of shores (variety man).