Though osu! Beatmaps can be very hard regardless of star count, I think a groove radar is needed for osu! and all of its modes, calculating the density of all the notes possible. It would appear as a pentagon-like radar that expands, shrinks, and changes shape depending on how easy or hard the beatmaps are in certain parts.
osu!
Rate: Shows how fast outer circles and how heavy spin markers move
Stack: Shows how many hit marker stacks are placed in
Spacing: Shows how far hit and roll markers are spaced apart in tight situations
Slide: Shows how many slide markers are in the beatmap
Spin: Shows how many spin markers are in the beatmap
Taiko
(unknown, since I usually don't play Taiko)
Catch The Beat:
Weight: Determines how heavy the basket carrier is, affecting movement speed
Duplicity: Shows how many fruit duplications, of the fastest rate possible, occur
Spacing: Shows how far fruits are spaced apart in tight situations
Juice: Shows how much fruits have juice or not
Avalanche: Shows how much avalanches are in the beatmap
osu!mania
Stream: Shows how many note streams, of the fastest rate possible, occur
Voltage: Analyzes the density of the notes
Chaos: Determines the amount of notes that aren't 4th or 8th
Air: Shows the simultaneous notes (2+ notes) count
Freeze: Shows the freeze note count
osu!
Rate: Shows how fast outer circles and how heavy spin markers move
Stack: Shows how many hit marker stacks are placed in
Spacing: Shows how far hit and roll markers are spaced apart in tight situations
Slide: Shows how many slide markers are in the beatmap
Spin: Shows how many spin markers are in the beatmap
Taiko
(unknown, since I usually don't play Taiko)
Catch The Beat:
Weight: Determines how heavy the basket carrier is, affecting movement speed
Duplicity: Shows how many fruit duplications, of the fastest rate possible, occur
Spacing: Shows how far fruits are spaced apart in tight situations
Juice: Shows how much fruits have juice or not
Avalanche: Shows how much avalanches are in the beatmap
osu!mania
Stream: Shows how many note streams, of the fastest rate possible, occur
Voltage: Analyzes the density of the notes
Chaos: Determines the amount of notes that aren't 4th or 8th
Air: Shows the simultaneous notes (2+ notes) count
Freeze: Shows the freeze note count