This was originally posted as a comment on The Verge's Jolla reveal article, but it's probably lost in a sea of comments there, so I'm reposting it here (with a few additions)...

Since not much information has been disclosed regarding the communication technology between the phone half and the "other" half -- whether it's using NFC, whether there's a physical contact that carries voltage -- please consider the ideas below as only a geek's wet dream:

• Keyboard flipcover
• Gamepad flipcover
• Sound backcover with stereo speakers that sound as good as the HTC One
• Solar charger backcover
• Induction charger backcover
• Extended battery backcover
• Camera backcover to improve night time photography
• Photo printer backcover
• Card scanner backcover
• Pico projector backcover
• DAC/headphone amp backcover
• Media extender backcover with HDMI out, optical and S/PDIF out, aptX, etc.
• Signal amplifier backcover for those with poor reception (similar to Wilson SignalBoost)
• MIMO backcover to increase Wi-Fi performance
• IR blaster backcover to turn your Jolla into a univeral remote
• E-ink display backcover for RSS feeds, notifications, reading books, etc.
• Magnetic backcover to easily attach your phone to the fridge
• Security backcover with physical encryption PIN (similar to http://worldsbestflashdrive.com/)
• Palmprint reader backcover so you can unlock your phone simply by holding it in your palm
• Celebrity promo backcovers that automatically inject custom wallpaper, RSS feeds, etc.
• 99-in-1 memory card reader backcover (you can never support enough formats! )
• Mirror backcover (you can never look too good )

It will be interesting to see how creative accessory makers get with this, but we also have to brace for the classic chicken-or-the-egg problem: in order for accessory makers to jump right in there has to be a market big enough to jump into, but in order to have a big market the phone needs to attract people with very cool backcovers that provide real added functionality (changing colors and ambience may be cool to show off at first, but it doesn't add any real functionality). To give the accessories ecosystem a little boost, I hope Jolla will come out with a killer accessory such as a keyboard backcover to show people what is possible.