I don't know as much about the long streams vs. fast streams since I'm not much of a speed player, but I definitely think smaller hitcircles and longer maps need to be rewarded much more.
I understand how the pp system currently accounts for smaller hitcircles, and that increasing your area is just like playing larger hitcircles, etc. But I can say in practice, it definitely isn't working like that. With how large the screen is, you can only make your area so big, and playing smaller circles requires more steady hands and better aim than the system makes it out to need due to how players snap. The difficulty gap is much more evident in CS6/CS7 maps + HR where aim is super hard but the aim tp is not very high, but I still think CS5+HR needs to be rewarded much more too. As a HDHR player, I've grown to dismiss CS5 maps as pp maps whenever I see them because they're always so much harder than CS4 maps that give the same amount of pp. I'm not sure exactly how to go about it so that the algorithm for it "makes sense", but I definitely think the scaling for points should be higher than the scaling for the circles themselves.
And as for longer maps, I've found that they generally do give lower pp than they deserve because of how much longer it takes to full combo them if you have some given percentage chance to miss at any point in the map. I mean, if you have a 1/10 chance at FCing a 500 combo map, you would ideally have a 1/100 chance at FCing a 1000 combo map, which means it would take on average 10 times as long. Obviously, if the map is really easy to FC, then it will take on average less than twice as long, but those maps won't give much pp anyway. Nerves are also a much bigger factor on longer maps. I don't think the map length scaling needs to be massive, but I think it should be at least noticeable.