I agree, there are countless both purely engineering and creative use cases for sidechaining!

My absolutely adored kind of sidechaining is spectral, that is, when it's not merely a loudness envelope of a source that drives the gain of a target, but when both are split into FFT bins and the envelope of each bin of the source drives the gain of the corresponding bin in the target.

That allows for carving out the target signal with the frequency response of the source, surgically. Works miracles is modern bass-heavy styles, along with spectral limiting.

There is one particularly amazing VST plugin at this, but I won't advertise here.