Basic sound FX such as amplification/limiting, especially low pass/high pass filters, and (dream scenario) slightly more advanced FX like chorus, delay, and reverb could greatly improve sound/music quality and give us more freedom of choice in how we can use the channels. Since we only get one dedicated waveform per channel, this is even more important than if were able to use multiple of the same waveform in the compositions.
For example, currently the sine channel provides one of the better deep bass sounds, but to relegate the sine wave to that job and lose it as an instrument seems a shame when simply adding low pass to any of the other waveforms could create a similar or better sound.
Likewise, the same FX applied to the in-game sounds could also open up much more range.
I was about to write in with a similar message so I'm glad I checked!
I think having the high/low/band pass filters and maybe panning would be enough to be able shape sounds while maintaining the semi retro aesthetic. I really like a lot of the limitations in the channels (more flexible than NES music but a similar idea), but obviously I wouldn't turn down chorus or reverb if they showed up haha
Polyphony, even two note, on each synth channel would also be a huge improvement. There's an inspiring creative charm to the technical bounds...but I'm greedy.
Would you take polyphony if you still could only have 5 total sounds going at once? I think I'd be willing to take that, just to maintain limitation while opening up other possibilities.