I know this isn't what you had in mind, but...
https://en.wikipedia.org/wiki/Project_Ara
Google's Ara got it wrong with it's slide-in modules. They put 'dividers' in instead of generic spaced square 'micro rails'. I.e. I should be able to install any module of any size in any orientation in any position. The device itself has 'space' for 3 module units by 6 module units.
All of these should be possible in any logical placement - even, 'hanging over the side': 1x1, 1x2, 1x3, 1x4, 1x5, 1x6, 2x1, 2x2, 2x3, 2x4, 2x5, 2x6, 3x1, 3x2, 3x3, 3x4, 3x5, 3x6.
A 1x3 and 3x1 module should be 'installation agnostic' and both fit into the same places. Also, every module should have the reverse 'micro rails' and contacts on it's back side so that multiple modules could ride in a double or more thickness device. Modules should 'touch' on the edges - no gaps.
Of course the primary assumption in the Ara project is a bit wrong now. It assumes that every function, GPS, WiFi, cell receiver, etc would each be contained an disparate chips in separate modules that could be added/removed. Now that those functions are progressively being consolidated onto fewer and fewer features (chips) on the motherboard, Ara has drifted into irrelevancy - as it is becoming cheaper to just install it all.