As the gold-standard of player customization, Foobar will allow you to build the player from the ground up, with the only restrictions being the elements you can choose from. The key feature is a layout builder that acts as a grid, so you can insert whatever element you want at any position on the player (filters, playlists, controls, search box, cover art, etc.). You can also choose from a vast array of formatting options for these elements (font color/size, BG color, etc).
Of the SliTaz players, Ario and Sonata are the best fit for me Ario has a set of filters on the top so you can have Genre, Artist, Album, Title and click through those to play. Would be nice if you could position them, but at least it works. Sonata comes up to the edge of usability and stops; you can filter down to the songs on the library tab, but when you double click the track it doesn't play...it just adds the track to a playlist and you have to go to the next tab to play the song. Gnome is also close to usable, but I don't like how it inserts thumb icons with the tag info (such as the Genre view) and doesn't allow you to turn them off. Most of the Linux players are focused on playlist creation. Playlist is an essential feature, but I don't like using them. I'd rather just filter to the song or album I want at the moment and double click it to play.
An ideal audiophile player for Linux would have the ability to build a completely custom GUI, a very light core, and a vast range of plugins for format codecs, VST sample codecs etc. One area of opportunity would be a player that can play DSD files natively (without converting to PCM). DSD may go down the tube, but it's size, sound quality and open source nature make it a format that a lot of expert would like to see rise to the top. There isn't much in the way of audio gear, software or music files out there at this point, but there are some companies working on expensive players for this format.