A special (or maybe even ranked) mod that gradually increases a songs bpm up to a cap by the end of the song (maybe 1.5x). I think it would be a pretty fun mod to mess around with. The main problem with ranking it would be that certain songs would be much harder depending on which parts of the song were hard so you could not find a good flat score multiplier, something like the tp system would have to be used to find the value of the mod for a particular map (if it were to be a ranked mod).