[sciurius]

-> it would be greate (sic) to be able to use collections (or other user defined parameter) as sync-filter.

While using collections for this purpose would be fine (at least for me, for now) it is just an arbitrary constraint and any other constraint (e.g. album, artist, ...) is equally valid. Supporting all could become complex.

I think switchable libraries (aka separate databases) is a better way to go.