I'm thinking {-s, -l, -a, -p} for {song, album, artist, playlist}.
Unlike the -e solution, it wouldn't make the command significantly more verbose than the default option.
You could also print a list of commands for the specific options (or allow for index based selection) in cases where there were collisions.
That's already mostly in. From the readme:
kew dir <album name> (sometimes it's necessary to specify it's a directory you want)
kew song <song> (or a song)
kew list <playlist> (or a playlist)
The directory can be an artist or an album, so there's still ambiguity there. But kew cannot differentiate between the two. It matches against files or against directories.
This here is even better, I love the minimal approach.