This probably sounds like a strange item for a build request as the market is obviously full of power banks for sale.
However, in general, I've found off-the-shelf power banks to be extremely unreliable. Either they fail within the first few recharge cycles, after about six months, or just outside of their warranty period a year later. Looking at customer reviews on the likes of Amazon, it would appear that quite a large number of people are having similar experiences, and over a wide range of brands. Not only is there the inconvenience of the device failing, but there often is no indication of an impending problem until you are in a situation where you really need some off-line power.
For example, last winter, we were hit by bad weather which flooded the city's main power substation, leaving most people without power and in the dark over most of the weekend. The first outage lasted for around 30 hours, then came back briefly, before going off again for around another 15 hours. There were then further 6h of outages as emergency generator supplies were reconnected back to the grid. Although this was a bit of an inconvenience, what was more inconvenient was that the deep cycle battery in my 12v 18Ah power bank decided to fail within the first few, which would have otherwise at least allowed me to read ebooks / watch videos / access files etc. on a tablet or similar mobile device throughout the power outage.
After that incident I purchased a Li-Ion power bank with a %age 'fuel gauge' and both USB and coaxial power outlets, however that failed after the first charge a few days later and had to be sent back.
Ben built a Li-Ion battery charger in a previous show based on one of the Maxim IC's however I was thinking that perhaps this could be taken further to create a universal power bank control circuit that could be attached to a battery of choice such as a small golf battery or large leisure battery, and add in some reliable monitoring features, especially for long periods of standby use. It looks like Maxim now do some interesting looking universal charger IC's with I2C bus connectivity for control/monitoring of a wide range of battery technologies
https://www.maximintegrated.com/en/products/power/battery-management.html