Fine-grained volume control
The steps at the lower end of volume control are too coarse, I can hardly find the proper volume when listening to podcasts before sleep.
Other apps, e.g. HERE, are able to implement volume control with very fine stepping with system-wide effect and no root access required. (that's just as an example to prove it can be done)
Beyond that it might be convenient to have a dynamic volume control depending on the current volume. Small steps when quiet, normal steps when already louder.