GaN is a pretty incredible material for power. It should be possible to build an amplifier that can scale from powering sensitive IEMs all the way to powering full-range speakers using GaN. Such amplifiers already exist, albeit without GaN; the Schiit Ragnarok is an example of such a device. With GaN, the gain stage can be smaller and more efficient, and possibly allow for even higher power.
Such an amp would be much closer to a pure voltage source, with more and more current for lower impedance loads before hitting a knee in distortion performance.