I've got remarks from several users that have trouble understanding the concept
of having players bound to different music sources.

I would like to be able to separate the Music Source from the player control.
The way to do this for me would be to keep one connection for NowPlaying (to
the server the current player is connected to) and one for the database itself.

I would consider this only for those players and servers that are known to each
other - that is the players that will be shown on a serverstatus query.

See also:
http://forums.slimdevices.com/showthread.php?p=426398

This enhancement would allow to play tracks that are currently not present on the server a player is connected to but on a different one. It's more or less what SC is doing with SN provided tracks today.
This would help a lot to coordinate between different servers providing different content, e.g. a library on a server and one on a laptop.

I don't know about the current architecture so I can't judge on how challenging this is and whether a "virtual player" functionality would have to be added to a server.