This is actually a great idea, this indicator would be much more obvious than a slidertick.
One thing is the startpoint could still be stacked like what Lyby said, if just part of the circle could be seen it's still hard to tell. So would it be better if we add a certain colour on the whole slidertrack (which must be clear in current rule), to indicate the coming speed up/down? Players could choose the color they like for indicating (i'll choose red for up and blue for down personally).
Also, the color depth could be a indicator to "how fast/slow the slider is"?