I know I am a little late to the party, and you all are already coming up with better formulas for star rating. Since a lot of you are mentioning about the pp you gain from DT, I would like to add one thing. It would be great if someone changed the mods we have for Taiko.
This one is going to be an unpopular opinion. HDFL is abusable through dual screening, and I myself wouldn't consider it to be a skill since you aren't actually reading anything and the OD has not changed. I'd say pp should be removed for HDFL. If not, only keep the pp for HD mod.
I'm sure some of you will agree with me on this one, as many people, including me, benefit from this perk: The extra star rating by using DT. Even though a lot of people will reduce their pp after this, I think that it is worth mentioning that DT gives way too much pp. (You can interpret it as NM giving way too little pp)
On the discussion of mods, I think it's time for a change of the mods itself. I think it seems like a good idea to be able to change the OD, SV, and BPM of the song, and these properties are all adjustable on a slider.
HR will be split into two. OD and SV. The OD slider can be adjusted from OD 0 to OD10, much like when you create a beatmap. The SV can be changed as well. (I can't decide a range because I've never actually touched the 1.4SV and always kept it that way) The BPM, much like HT and DT, can be changed on a slider from 0.5x speed to, lets say, 2x speed.
Only the OD slider and BPM slider can make a change on the pp you gain from the song though, and the SV does not matter.
With these suggestions mentioned, I would like to ask all of you whether you think it is a good idea or not, and if you think it would benefit the pp system or not. If not, do you think the slider can be implemented anyways, just for Taiko players to have fun? (I've seen a lot of people make the same map except speed it up by 1.2 or 1.25x just because it's more fun that way, and thought this feature would be better for taiko)
TL;DR: HDFL shouldn't give pp. DT gives too much pp. We should be able to change the song's OD, SV, and BPM, but only OD and BPM will change how much pp we get.