Wavetable support perhaps in the format of 8 bit unsigned 64 waves, no interpolation needed as you can bake that into the wavetable, like what is found in classic waldorf synths or PPG wave.
Reasoning - it would be another powerful tool for sound / synthesis which could keep file sizes small without giving up audio quality / vision. Custom wavetables can add a huge range of sounds but also more flexible than a PCM sample as far longer evolving sounds are concerned.
It's already in there, actually, but it looks like I never added an interface. 
I implemented that when I got religion about aliasing so I could switch to band-limited wavetables in the waveform type synths before those reflected overtones become audible. I started on a tool for building wavetable synths but got derailed by more pressing stuff and guess I forgot about it.. It's likely that I started thinking about file formats and looked into using SoundFonts and then got a bit overwhelmed.
I don't have an issue on this currently so I'll open one and get back to it when I have a chance. I'll have to finish my Playdate keyboard so I have something to test with. 