Ah, these wonderful concepts like the Other Half or Ara, providing modularity to small devices by such strange and exciting methods!

A long time ago in a universe far, far away, I recall mobile devices commonly used to have something called a "PCMCIA slot" (also known as a PC-Card slot). The PCMCIA card worked very much like an expansion card on a desktop computer, only miniaturized and designed for the power and space constraints of a laptop or other mobile device. And you could build space for such a card into a device's frame without making radical design decisions.

I'm not saying that the PCMCIA standard itself could be used in a modern phone; certainly, the cards are way, way too big. But I don't see why a small slot couldn't be carved out into the side of a modern phone with an OH- or Ara-like connector; something that might be deep enough to fit a small bit of electronics directly, and certainly allow a larger peripheral (or dock) to be connected when the user would like. (In my opinion, the existing OH and Ara concepts themselves demand way too much from a design; e.g., I have no clue how you'd be able to match the existing OH concept to the new Jolla tablet...)