I'm working on prototyping the fading thing right now. I'm trying to make it so that when a block fades in completely, it becomes solid, and when it fades out completely, it ceases to have any effect. This might introduce some ambiguity--is this partly-faded block solid?--and the prototype will tell us how much of an issue that is. I might have fading-in cells tinged green and fading-out cells tinged red or something.
The plan for now is to have different speeds for different levels. A level where you have to try to make sense of chaos, like the prototype demands, works better with a slow rate, but any time you'd want to ride a glider, you'd want it to move faster than the ones in the prototype go.
I like the fluid time thing, and it would probably go really well with the fading part! I have no idea what the code would look like for the fading, but that's okay. The gimmick is used to really good effect in
Idealism and Braid's fourth world (which you can try out in the
free demo). I probably won't go for that particular gimmick, again, but I'll try to make the code lenient.