This is very interesting. I don't agree with "scaling pp for other keymodes" though. 7K will be biased anyway because it hasmuch more hard stuff ranked and good system won't have to scale anything to get accurate ratings. Big props for all the other stuff, I hope pp guys will read that many timesRegarding the difficulty system: Of course, a perfect difficulty system would not have to scale for other keymodes - but that'd require tons of effort and while it is possible (completely different numbers for different keymodes, different factors involved), I can't quite imagine it happening any time soon. I guess you can see this as a way to fix the difficulty system as it is at the moment and an additional step to reduce bias towards higher keymodes, before revamping it to a difficulty system that is able to encompass all key modes accurately.
Anyway, you're right when you said that the 7K mappool has more hard maps than 4K (and other modes), and it will be biased because of that. Regardless though, in an ideal scenario where each mappool for each key mode has the same amount of hard maps, I do think it'll be completely unfair if the system gives significantly more PP to a player who plays 7K than one that plays 4K although both have the same score and both files require around the same amount of effort, because the difficulty system will on average higher-key maps higher than lower-key maps.
Thanks for the comment, much appreciated