I really like the idea. Especially if playing with relatively new Osu-Players even HD HR is boring, but i play like this anyways cause i enjoy playing with them. (OT: I think here is also a big source of high ranks iin easy maps, which makes it almost impossible for newbs to get an appropiate reward for fcing a map)
I think an easy solution to the length problem is: Not all Maps are allowed to be played like this.
With some preprocessing there can be easily produced Tags for each Map, that groups (like said before) the Diffs with similar length. (+-5% length maybe, as some Maps are mapped almost with same length). Those Maps can be played in this "choose you own diff"-mode. I think that would be most of the maps. (Ive just watched through some random maps and most of the diffs dont have more than 5 sec difference, which is acceptable i think) Also the remaining waiting time is at least filled with music! It is no plain waiting, as there obviously is mappable content in the remaining time, which is a huge difference to the problem within the DT/HT thread.
The preprocessing also offers a solution to the skip issue: Players can skip to the earliest skip in all played diff. Most Maps skipping to the same point anyways.
So the problems adressed are only a problem for a minority of the maps (have to be confirmed, only made random checks), which is then unplayable in this mode.. So what...
It is obviously something that has to be toggled. Some people looking for the challenge and competition. Some only want to have fun together.