I see two major problems:I agree. Something like what Osu!Mania has would be nice. Maybe 0.9x (or even less) for non-standard over 1.0x speed and .3x for anything under 1.0x like you said. Then DT and higher would be at 1.12x points.
1) Making more plays unranked just isn't going to fly. Peppy would have liked even autopilot and relax to be ranked, but that proved too insane to keep.
2) Too prone to min-maxing. Especially when the score multiplier is tweakable.
Which means that the most acceptable form I can see would be one where the speed cannot go lower than the current HT, and the score modifier isn't graduated... slower than normal gets the HT modifier, faster than DT gets the DT modifier, and in between gets the standard 1.0x. But I still think min-maxing will be problematic... there are a good number of songs I'd like just a bit faster to get them out of an awkward BPM and into a more comfortable one. Which means that maybe the interval from 1.0x speed to 1.5x speed should have a multiplier less than 1.0x, because it can be used to make things easier.
I'd still like it as an option, even if it is unranked.