Well, the concept I originally suggested (and bear in mind that you're in no way beholden to it if you have an alternative idea) was that the planetoids themselves would not move,
Mario would--specifically, he would fly through the area at high speed, dodging assorted obstacles in his way. During his flight, he would be able to latch onto small, sparkling planetoids, whose gravitational pull would allow him to stop for a moment before continuing his flight. As a very rough comparison, think of the sections in the
Hydrocity Zone of Sonic 3 where Sonic is being blown along underwater by a fan, and can catch on to various poles to briefly stop himself, only developed into the basis of a sublevel
Of course, the
real essence of the stage is the idea that the normal rules of logic and physics as Mario understands them are starting to break down, giving him new-found freedom of movement, and allowing him to do all sorts of crazy things as a result. This could (but doesn't absolutely have to) translate as his being able to fly, which is a good representation of such an idea, and is apparently an activity people frequently associate with dreams (though I honestly can't remember a single dream where I did so myself, personally). Flying would would likewise be a logical approach considering the
previous phase, which has Mario bouncing about as a sort of proto-flight, and ends with him being launched forward into the air in anticipation of full on, unhindered flight in the next area, but again, you should feel free to exercise your creativity here. The important central idea of this phase is simply that the normal rules no longer apply to Mario, and he can now do things that were never possible for him before. See where that takes you.
As for sublevels, just claim any free one in the appropriate thread. You could request special ASM for this area if you like, but remember that there are quite a lot of existing blocks, both already inserted in the ROM or existing in the block section, which could also be useful without the need for any unique code. It's all your choice.