[TC_Halogen]

also, absolutely yes to anything that is of ascending + mixed difficulty but freely open like the dan system; it works super well in other rhythm games and would work nicely here too

honestly, the tier point system could also use a bit of a refresh (not necessarily just song-wise but also with graphics that makes it work - hell, something like that could even be "seasonal" of sorts)

EDIT: another system to look at as well could be something similar to what Life4 does, where you could have various named courses and assign a dan/rank based off of overall performance on that course so that players can appreciate all different types of songs/varieties but be rewarded based on their performance - example of the idea can be found here

EDIT 2: also to be approachable to newer players, you could adopt a sequence of "kyu" level courses before the dans, which could lead considerably lower level players into playing more to increase their overall ability -- i'm not sure what the scaling will be but splitting over a larger set of ranks makes the curve a bit easier to handle earlier on (you can let loose further up since players at that point are already engaged and likely training to continue on)