The dimensions are odd - not enough in one dimension and too much in another. At 35.8cm wide, it's less than most airlines' 40cm maximum width, so it doesn't maximise potential internal space. And its depth of 22.9cm is too deep for the likes of Ryanair.

I would like to see a built-in Li-ion battery that cannot be quickly removed without tools, so that airlines cannot choose to gate-check it, but cannot deny it entry to the cabin.