In a typical hike, not only do I care about weight and capacity, but also recharge speed from AC.

I often will be stopping at a cafe for lunch, and in the 45 mins I'm there I want my phone to charge from 15% to 90%, and I also want my battery to recharge from 15% to 90% to give me another 3 days hiking before the next recharge stop. That involves carrying a dual-output USB adaptor where both outputs are fast charging, two cables, a battery bank and my phone.

Thats a lot of stuff to carry, when someone would ideally make a single AC adaptor with a built in battery and cable such that when plugged in to AC it recharges both itself and an attached phone, and when unplugged from AC it discharges itself into the phone.

With clever design, some of those bits of electronics can be combined and casings and heatsinks shared making the whole setup smaller, lighter and cheaper. By integrating the battery charging logic into the AC adaptor, the temperature of the adaptor and output towards the phone can be used to adjust the charging speed to maximize use of the flyback transformers saturation current.