SUMMARY
In the moment it is only possible to seperate items by album, month, type and faces. I would wish to be able to group by more custom attributes.

Use case: I would like to be able to seperate items by some exif data (e.g. Camera Name) or tags.

REALISATION
One out of following possiblities (probably from easiest to most advanced):
 - by enabling some fixed exif lookups: Title, Camera Name, Autor, Rating would be interesting for me
 - by enabling custom exif lookups, that the user can configure which exif tags are possible
 - by enabling tags as seperation method (sufficient for me, but probably difficult to handle multi-taggings)