Just a few quick Yes/No questions (I didn't read the whole thread, so they may have been answered already)
1. Do you gain a different amount of PP depending on where you hit your 100s/50s (i.e. near the end/beginning)?
1.a. If yes, is it simply based on the combo count, or is it something else?
1.b. If no, are you still rewarded by highly weighted maps even if you perform poorly on the actual "highly weighted part", but play perfectly for the easier sections?
2. Does a sliderbreak count as a miss? (i.e. would a 99% S with a sliderbreak be worth significantly more than a 99% A with a miss?)
3. Are spinners included in the PP formula? Also, would Spun Out affect your PP gain?
4. On the osu!tp info page it mentioned that the speed value is affected by map length. Does this mean (in general) longer maps such as marathons will reward more PP than a shorter map?
Suggestions:
1. Could readability also count towards map difficulty?
Maps like
http://osu.ppy.sh/s/7671 are known to be hard for being incredibly confusing (although I suppose people can just memorize everything, so the added value should be somewhere between hidden and flashlight).
2. Although the vast majority would agree that a map with a higher AR is much harder and hence HR should be rewarded as such (also due to the increased values in OD, CS and Drain), most people I have spoken to would also agree that an AR lower than the recommended map AR is also much harder. Hence the order of difficulty would somewhere be along the lines of HR >>> EZ > No Mod.
My suggestion would be to keep the PP reduction for the lowered OD and Drain, but re-evaluate how AR and CS contribute to map difficulty.
3. Another suggestion is to make sliders contribute to accuracy to... some extent. Although I agree it's a free 300 most of the time, getting a 300 should still reward something, and not getting a 300 should give some form of penalty. This map (
http://osutp.net/scores?bid=66941) probably isn't the best example, but as of now the PP rewarded for HDDTHR is worth less than a No Mod SS on most 4~5 star maps.
4. One more suggestion is (if possible) to calculate the PP gained from a map based on the score which gives the most PP, and not simply the highest score. For most maps, a No Mod SS is worth significantly more PP than a 97% HD score, despite the latter having a higher score. In such cases, the No Mod SS should be considered as the player's "best play" rather than the HD score (only in terms of PP calculation). A score should only be overwritten if the new score rewards more PP.
Thanks for reading!