I just want to add UPS functionality to some of my flock of raspberry pis, and with an emphasis on portability.
One of the ways to get this is with certain USB battery banks. I have one here that is capable of continuing to provide output even while you plug the pack itself in to charge. This works, but is quite cumbersome and bulky.
What I'd like is a portable circuit that has a battery JST plug or just solder pads, so I can choose what size lipo to use, and most importantly is designed around having reliable power delivery to the pi. An elegant way is to allow for this battery board to charge the lipo whenever the pi is powered via microUSB, and to serve power to the pi as a UPS whenever it becomes unplugged. Preferably, the entire contraption can mount cleanly on the underside of the Pi to enable transparent full use of GPIO pins, and be a reasonable form factor for both full size Pi and Pi Zero.
This way, I can use a very small lipo with it and it will still serve a useful purpose, because even with only one hour of reserve power available, which would only require a tiny battery, it does provide the ability for me to hot-disconnect from power without shutting it off!
Having a JST plug would be excellent as well as it would mean that the battery can also be hot-replaced as long as the pi is plugged into USB power.
Furthermore, the functionality should be seamless so no configuration or limits on which Pi models (in particular the Bluetooth-having versions behave differently in many cases) are used. It really really preferably is a transparent hardware only modification.