Even though I understand the point very well and how some maps could benefit from it, I'll have to disagree with the need for it because of a few things;
There is no shortage of in between taiko specific maps and difficulties, you just have to look for them or alternatively ask somebody for map suggestions.
Converted maps with an object density equivalent of said in between difficulty far more often than not don't really got unplayable slider velocities. I can with quite some certainty say that about 180bpm 1,8x isn't actually particularly high, and just about anybody who finds muzu diffs as too easy will be able to play it with no trouble.
Everyone prefer different SVs, or rather, scrolling speeds. I personally and quite a lot of others as well can't stand 1,4x and are generally forced to always play with HR. It's already slow and no challenge even with DT/HR and I strongly disagree with making things easier when they're not particularly hard in the first place.
That said, for mapping difficulties specifically to work in both modes this could come in handy, but it would make things by miles too boring and easy if this suddenly popped up and enforced the 1,4-1,6 rule on every single map. There's already a roundabout to this, and that's simply making a copy of your map and setting the slider velocity to something reasonable, and as such the point of this feature is pretty much no more than "make maps easier" which I can't agree with at all.
Rather than making everything easy, get an easier map to play and let those who find it to be just right enjoy it.