Built-in flip out USB connection is lacking. The smaller the Powercore the more the USB socket becomes as a ratio of volume and the cable becomes nearly the same size, so I think either built-in output plug or merger of input and output via USB-C.
At the opposite end you have the thermal density problem. In large devices they solve this via very thin batteries inside a rigid case with very good device battery life
So what I see is a squeeze. At the low end it has to become a powered cable. At high end it has a futility where it's just Wattage of a charger.