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...