Well recently I needed a scrollbar that supports 64-bit integers but obviously Qt only provides support for "int". Since the scrollbar is going to have to be rounded depending on the number of pixels on the screen I was thinking why isn't there a @double QAbstractSlider::percent() const@ and @void QAbstractSlider::setPercent(double)@ provided ? Not really sure what would need to be done in order to get this into some release of Qt as I think it is pretty useful to have.
You can start by talking about it on the development mailing list. If everything goes well you can start writing a patch to Qt and submit it to gerrit for review (open a feature request first is also a good idea so it helps keep tracks of it).
Alternatively, you can also create a Q64BitSpinBox (with a better name than that) and provide it as an add-on
Happy coding !