Today, a new project has been registered on Launchpad, called Rhythm-e. The project was registered by Ammonky - who's also behind Nautilus Elementary. No code has been published yet (update: the code can be found HERE), but you can get an idea on how it will look:
It's not finished yet but it's a really nice start! The above screenshot is not a mockup - it's the actually Rhythm-e (screenshot posted by Ammonkey HERE). Even though not implemented yet, the plan is to get video support as well as an advanced Clutter View into Rhythm-e!
Update: I was able to compile Rhythm-e. You can see all the new features in the screenshot already posted, there's only one thing to note: you can toggle the menu using F8, just like it is with Nautilus Elementary. Here is a screenshot from my computer (you can of course re-enable all the elements you see disabled in the screenshot):
And here is the original Rhythmbox 0.13.1:
Without any connection with Rhythm-e, I've also found some Rhythmbox Elementary mockups posted @ Gnome-Look:
The above mock-up is a mixture of the Rhythmbox features and Nautilus Elementary user interface. You can see all the screenshots, here.
This leads us to the question: does Rhythmbox need a new interface? Also, what do you think of Rhythm-e?
