In light of the in-flux star rating overhaul, I find it necessary to bump this thread. It's about time that more "official" bands were added, so that difficulty can be distinguished more easily at a glance.
Ideally, I'd like to see a 6- or 7-star default, which would scale nicely from [Easy] through to [Expert]--at the moment, the recalculation is shoving [E/N/H/I] to less than three stars, which I worry is going to become set in stone. This would make for horrible differentiation. I fully believe that keeping a "classic" spread of 1-5 would cover a tightly focused E-I spread pretty much as-is, while any maps in excess of five stars would be labelled as [Expert] or [Extra]--one well-defined band above [Insane]. For any maps which go even further, then could either cap at 6 stars as an indicator of how ridiculous they are, or they could creep towards 7 as a way of defining them even further. Such a difficulty could have a [!] icon to reinforce this.
While ten stars would allow for plenty of scalability, there is surely a limit to what mappers can do in relation to a piece of music. The last thing that this game needs is a doubling of stars, because having this amount of breathing space would invariably make lots of people create something for the sole purpose of attaining 10 stars, instead of concentrating on fitting the music. I can't forsee a compelling argument against this logic, because my observations tell me that this would almost be guaranteed to happen.
The ultimate goal of this sort of change would be to preserve the current 1-2-3-4-5 spread for clarity, then allow a bit of space to go beyond that as a reflection of current playing standards. The actual star rating would ideally be accurate enough that 1-5 becomes a fair reflection of [E-I], so nothing would be broken in this ideal world.
DEEDIT: After a bit more thought, allowing for even more stars could probably work if the calculator really does give a fair reflection of difficulty jumps. My observations are basically this: maps like 'Airman' and 'Big Black' seem to be maybe a step or two above a higher-tier [Expert], similarly to how the lower diffs are separated. Feel free to prove otherwise--this is just how I break down the difference in my mind.