The Mega65 can take lots of RAM through intelligent bank switching, but an 8-bit processor still has an address space of 64KB and works on 8 bits of data at a time. Doing arithmetic on 16-bit integers is possible, but not convenient for a programmer and neither is it efficient to emulate 16-bit arthmetic with 8-bit arithmetic. Considering that the C64 could do 4-bit samples with the CPU doing all the work, 8-bit with DMA playback will be already an enormous improvement in sound quality.
The Amiga had 4 8-bit audio channels. Most MOD file formats use 8-bit samples and they could sound great. 14-bit sample playback on the Amiga is possible with tricks: By using the volume control in a clever way and by combining audio channels, you could achieve 14-bit. But that was not the way most software did use the Amiga. Most games as you remember them used the 4 audio channels as they were intended: 8-bit audio.
For SID improvements: A SID waveform is pointless: The SID can already do that. Take a triangle waveform and apply a bandpass filter on the frequency of your note. You get a beautiful sine wave. If waveforms are added, we should look at waveforms that really enhance the audio capabilities.
Does a reverse saw have a different sound than a forward saw? If yes, I think it is a potential candidate as usefull addition. What do you mean with super saw?