Clearly with little information to go on, and nothing more than speculation regarding the means of connectivity, I thought it would be a good idea to start a thread to open discussion around the sort of "Other Half" concepts that could be used with the upcoming Jolla Handset.
Certainly videos of the "I Am The Other Half" event and associated interviews have indicated a range of existing ideas from lens and flash attachments to theme and content options.

Of course the first to come forward for those of us who still hold the N900 dear to hearts (and hands) would be a qwerty slider "half"

As my graphic design skills are about as hopeless as my coding skills, I'll be more than happy to leave it for others to offer renders!

If nothing else, this thread may provide fertile ground as a rudimentary "suggestion box" to Jolla for further consideration.

As I said elsewhere, e-ink display and solar panel w/battery (so that we can charge the cover seperately from the phone)

Another idea is game controls like nintendo circle pad and rear touchpad a la psVita.

I'm intrigued by the idea of a clambook style "other half" where the connection point allows the handset to become a trackpad/mouse. After all, there's no reason the other half needs to be dimensionally proportionate.

Assuming a wireless screensharing capability and bluetooth HID for the keyboard, this could push SailfishOS right into the tablet market as well.

Ethernet-Port with Power over Ethernet for charging. So you could take it into office and use it for VoIP fun and at the end of the day it is charged.

Headphone amp and with the space left possible extended battery.

Keyboard slider.
Camera lens with cover.
Pressure sensitive drawing screen packed with stylus.
Audio OH with analogue and digital input / output and a mic appropriate for recording.

first of all, keyboard slider

if possible, drawing tablet

solar charging cover

Infrared . Extra lens . Fm tx .

Just an opinion, but...

I think the 'Other-Half' deserved its own thread considering the currency of the topic, the ambiguity followers are facing with the lack of information, and the potential realisation of the imaginative OH.

Why scatter a focussed topic? It can be so difficult and time consuming sometimes to navigate the thousands of posts under one general thread!