Wait, what?
In reality, we pretty much reached the difficulty peak a year ago... osu! is severely limited difficulty-wise by its one input to worry about (hence, z/x). Maps have already been made with lots of 1/4s. Let's see...
Heatman
Biggest Dreamer (Kasada's Insane)
SSII
Kanbu
Agony (Insane)
That one Max map that is just silly
And the list goes on...
Those are pretty much as hard as you're gonna get. Oh right they're old, especially SSII, Kanbu, and Agony. If you think about it, the maps nowadays aren't that much harder.
Let's take Heatman. Not many people have S'd that map. That's because the opening is one of the most ridiculous openings for a map in history. This is because Heatman was made as a joke and to prove a point. You can't really get stupider than the Heatman opening.
I haven't seen very many maps stupider than Heatman. This is probably a good thing.
Taiko? We still have a long ways to go there. 'Course, we'll have to beat the chart that is mostly a single 1/4 stream. Just use a 1/6 stream instead, problem solved. ~ Honestly, if you want more challenge and want to utilize your 1337 keyboard skills, I recommend Taiko or any other input-based music game. Even Guitar Hero is harder than osu! (IMO of course) due to the complexity of some charts.
So yeah, it can't really get extremer. BTW, I know some osu!ers who don't like Stepmania for that very reason. ;/
Dear OP: the hardest osu! maps have been this hard for over a year now. Please deal with it. Thanks, everyone else.