I recently entered the “fabulous world” of multichannel audio routing/output and I found that a common problem for almost all platform is the default audio mixer limitations.
Nowdays almost all soundcard (embedded too) have multiple output channels and an increasing number of users does use them for many different purposes.
Of course there are 3rd party “wrappers” for almost any OS, but many of them are difficoult to use or introduces bugs into the hosting system.
So it would be great to have advanced options/features in the default one, IMHO.
Here are some (well known also) interesting projects that may inspire:
Hope that inspire.