At one point, Super Mario Galaxy 2 was Super Mario Galaxy 1.5
“As soon as we completed Super Mario Galaxy, we thought we weren’t able to include a lot of the new ideas that we really wanted to. As soon as the project was over, we started making something that we might call version 1.5. We did things like moving or adding stars to levels that were already available. As we made progress with this 1.5, we realized that there were too many things to fit into a version 1.5. That made us realize we should start from scratch and make Super Mario Galaxy 2. That’s how we started working on the game…. Usually we wait until the advent of new hardware, but for this time around, because there were so many ideas we were not able to include in the first volume, we thought we should make a sequel to that.” – Shigeru Miyamoto
I don’t think something like Super Mario Galaxy 1.5 would have sat well with fans. It’s definitely a good thing that we’re getting a full-blown sequel with new levels, new ideas, and new mechanics.