My ideal delta indicator shows both time delta and instantaneous speed delta. The iracing delta bar kind of does this through the colour of the bar, but it's not perfect, as 1) it's tough to discern between varying degrees of green/white/red, and 2) if the time delta is low, there isn't much of a bar to see.

Better would be if you could configure the visual portion of the delta app (the red or green bar/dial) to optionally indicate the speed delta (red for negative, green for positive, size of bar/dial indicates amount), while the time delta would just be shown by the large number in green or red, as it is now. So basically in the same footprint as the current overlay, you could get both pieces of information, rather than the same thing in two different ways. (I can see how some people would like having the visual indication of time delta along with the number though, so ideally it would be optional to have it either way.)