STFT with varying window lengths (like the continuous wavelet transform)
This is sometimes done when creating a log frequency scaled spectrogram (which might better match human time versus pitch perception).
One issue with using multiple sizes of STFTs it that this results in multiple FFT outputs for the same graphical plot point (frequency vs. time).  So you have to decide how to select from, scale, mix, interpolate, and/or cross-fade the overlapping FFT information without adding (more) visual banding artifacts, and in the frequency domain as well as the time domain, where the resolution difference problems can be orthogonal.