I don't wanna deny anything but since overmap is not unrankable, it's not that difficult to reach 30M points on your average high bpm song. Especially since higher difficulty settings change the maximum score by so much it's just silly (try hp9+ar9+od9, and maybe even add cs5 on your regular 20m song and see the score skyrocket).
Also 3 minutes is way too short, nearly every full size song is over that, if not even 4 minutes.
The note density seems like a good option but that would first need to be defined exactly so it's clear to everyone, however you can still create instability in maps, having some parts super dense while others are not.
I know that people want freedom, and I am not really against it, however what people never think about is how a rule or a system can get abused. Even if there is only one case of abuse, it's a fail system. It needs to be perfect.
The current system might be restricting, but there is no abuse.
Currently the Marathon Rule is 6 minutes, and I think that's fine, however just because some songs are not exactly 6 minutes and are slightly under, in my opinion they shouldn't get prevented from getting approved, if the map really deserves to be. This is another matter though.
The original Approval rule was 4:30 and/or score above 18~20m, however that got nuked because, if I remember correctly, peppy decided (and posted somewhere on these forums, can't find it now) that all maps should actually aim for rank and not for approval. Approval was only meant to be for really special and unique maps, and true marathons, however longer songs were considered to be a marathon for some reason too.