One thing I noticed that I do upon subsequent playthroughs is avoid getting more than 1 medal per stage. In Sonic, the gap between stages being a special stage is a fun change of pace, but that gap lasting 2 or more stages can be a bit too long.
Not sure if there's a solution to this , but its my one real bit of criticism I have to offer.
They look great as is (sans the fire level's lack of visual clarity), but special stages in Sonic have always set themselves apart from the main game visually. The stages in this simply reuse theming from the existing levels. Are there any plans to change em up visually at all?