I'm not a music guy either, but it seems to me you should Keep It Simple Stupid (KISS) : Don't bother with fancy panning modes, when you can achieve the same with two audio channels. Better to just have as many h/w channels as you can easily accomodate (8 at least, 16 would be cool, but 32 might be overkill), and then let the CPU handle any complex effects.
IMHO the "Amiga way" was to have flexible/semi-programmable hardware, rather than to hard-code specific effects into the hardware. So any semi-general functionality which would allow you to reduce the CPU load (say some clever DMA modes) for certain effects would be great (and maybe programmers can find ways to use such functionality in ways not imagined).