This hinges on a philosophy to uphold gameplay higher than providing as many players with their favorite song. Many may disagree with that, but to me it makes sense to make great maps that fit songs rather than cater to as many people as possible.
Difficulties are ranked one at a time. Ranking one difficulty at a time allows the mapper to focus all of their efforts to make that one difficulty with as much quality as possible. Each spread starts with one ranked difficulty which the mapper can later add onto. Mappers don't have to worry about a difficulty not fitting with the rest, and doesn't have to worry about putting equal effort into other diffs. Spreads are seen merely as grouping for maps under same song made by same mapper.
On one side of the extreme you have easier diffs for every song, undermapped or not, but a deficiency of harder diffs overall. On the other side of the extreme you have a surplus of harder diffs overall, overmapped or not, but a lack of easier diffs for every song. Currently there is imbalance toward the first extreme. This would change towards the second extreme.
In order to ensure there is not a surplus of any difficulty, players have a certain quota they need to fill before they can map harder maps. Rank 1 easy diff before ranking a normal diff. Rank 1 normal diff before ranking a hard diff. Rank 1 hard diff before ranking an insane diff, and so on. It doesn't matter which song the mapper ranks the diff for, so long as they rank an easier diff before a hard one. Doing so allows the mapper to choose the most appropriate song for the difficulty.
This would undoubtedly makes players wish there to be easier diffs for many of the maps that would only have harder diffs. At least there would be an equal representation of diffs in ranked and maps that fit songs. They can always map and rank easier diffs themselves if they want.
Difficulties are ranked one at a time. Ranking one difficulty at a time allows the mapper to focus all of their efforts to make that one difficulty with as much quality as possible. Each spread starts with one ranked difficulty which the mapper can later add onto. Mappers don't have to worry about a difficulty not fitting with the rest, and doesn't have to worry about putting equal effort into other diffs. Spreads are seen merely as grouping for maps under same song made by same mapper.
On one side of the extreme you have easier diffs for every song, undermapped or not, but a deficiency of harder diffs overall. On the other side of the extreme you have a surplus of harder diffs overall, overmapped or not, but a lack of easier diffs for every song. Currently there is imbalance toward the first extreme. This would change towards the second extreme.
In order to ensure there is not a surplus of any difficulty, players have a certain quota they need to fill before they can map harder maps. Rank 1 easy diff before ranking a normal diff. Rank 1 normal diff before ranking a hard diff. Rank 1 hard diff before ranking an insane diff, and so on. It doesn't matter which song the mapper ranks the diff for, so long as they rank an easier diff before a hard one. Doing so allows the mapper to choose the most appropriate song for the difficulty.
This would undoubtedly makes players wish there to be easier diffs for many of the maps that would only have harder diffs. At least there would be an equal representation of diffs in ranked and maps that fit songs. They can always map and rank easier diffs themselves if they want.
