Would it be possible to display the estimated charge time?
Yes and no. It is definitely possible to display something, the problem is this something will not be very accurate.
iOS reports battery charge in 5% intervals only and there is no information about the progress between these intervals. As a result, the initial estimation can only be based on the current percentage. However, this information is not enough – you need to know the “speed” of charging to predict the charge time (the speed depends on whether your device is connected to your computer or charger, whether some apps are running and consuming a lot of CPU, whether the display of the device is on, etc.).
Some rough estimation of the charging speed could be obtained after charging has been running for some time but this is not how people want to use the estimated time – they want to have a look at the value at the moment when they connect the cable to their device, not in the middle of charging.
I have tried several apps which try to estimate charge time but none of them works reliably. Instead of providing wrong and unreliable estimation, I have decided not to implement this feature at all.