I just happen to stumble over this idea. Everyone knows the Double Time Mod and the Nightcore Mod right? I just happen to think like, " How about the song starts from its original BPM/ speed and gradually speeds up to the song's BPM when it's in Double Time. i.e. "Kusaregedou to Chocolate" has a BPM of 260, if this mod is selected, the song will start with its original speed, and at some point, the speed will increase.
What about if the song has a long interval before the circles appear? Take the song "Yellow" by Hatsune Miku for example, also in this mod, the speed will not increase unless the main gameplay (hitting circles and sliders etc.) has started.
What about if the song has a long interval before the circles appear? Take the song "Yellow" by Hatsune Miku for example, also in this mod, the speed will not increase unless the main gameplay (hitting circles and sliders etc.) has started.