I had a similar thought and was about to create a post with that same suggestion. A modular powerbank would mostly add value since it allowed me to swap butteries and charge the depleted one outside of my fairphone. As it is now, I could buy a spare battery and swap, but I’d have to charge the empty battery later inside the phone, basically doubling charging time. With OP’s suggestion, I could oursource Fairphone charging into said powerbank altogether. Looks amazing to me.