What about the other half having wheels and an engine?

Slot the phone into an NFC enabled rectangular receptacle on a car dash for a multitude of uses. Hands free, Sat-Nav....... Could even do away with the bulky old car audio system.

The Other Half: waterproof case for the phone, with UHF CB radio and small hardware keyboard. It would be difficult but not impossible. Right now, the radio handheld units are considered separate product, used in extreme situations far away from cities. If OtherHalf-HandheldRadio-Waterproof was done, with an easy way to program certain buttons to make cellular calls, radio calls, send SMS, take photographs, look on a GPS-Map, it could be very popular. Cheap, since the OtherHalf has no programming - only waterproofness, dumb radio and dumb hw-keyboard; all programming is done on Jolla-Phone. Though, a 5W radio could probably use the phone battery quite quickly, so it's better to feed the radio from AA batteries of the OtherHalf or whatever is easy to carry, change and recharge.
Best wishes.
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Per aspera ad astra...

It'd be great to have a hybrid resistive-capacitive screen as an option...

Just to remind you we are still living in a real world, so calm down.

But imagine an ex|phone combined with the Other Half
(sorry for having only a german version link available, but I think you get the impression )

maemo, resitive touchscreen, slide qwerty, quad core?

Maemo - possible, if we get all components (drivers, etc.). It might be easier to port Maemo libs to Sailfish. (Maybe HF would get permission to redist some of it)
Resistive - impossible, 'cause many apps will take advantage of multitouch gestures. A hybrid touchscreen is the only way out.
qwerty - would love it!
quad core - would love it!

I am curious about idea of hybrid touchscreen. Since most people need not more than six fingers on the same screen, wouldn't installing six resistive touchscreens side-by-side (for example, in a 3x2 grid) be enough for most gestures? Well, for precision - since sometimes the gestures require the fingers to be close to each other - we could have fifteen-sixteen resistive touchscreens in a 3x5 or 6x6 grid.
Yes, screen calibration would be complex as hell with many separate sub-screens. But you could develop a UI similar to a game or something else as entertaining.
What other advantages does capacitive screen have, beside multi-touch? Would this hybrid screen be the best of both worlds?
Best wishes.

Capacitive one is more sensitive.

But it has some downsides - you can't use it with gloves, you need a special stylus (a pencil/ballpoint/guitar pick/book won't work for manipulating the stuff).

Beside this, I'd prefer a plastic screen to a glass one - I don't want my phone to have a broken screen after one falldown.

The idea about grid isn't a great idea: imagine clicking the media player play with 16grid...

More info about hybrid one;
http://www.slashgear.com/rim-develop...creen-0551295/

Another thought about the grid concept, I believe neonode had something like that patented where it used infrared sensors to detect there the finger was and when it crossed boundaries.

best of both worlds is capacitive with wacom. It even allows hover.

Being super-accurate with wacom pen on glossy glass is a bit harder, and matte glass is not very good for finger swiping but I think a special stylus material with a bit of friction could solve that.