Would it be possible to display the estimated charge time?

Yes and no. It is definitely possible to display something; the problem is that 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 estimate 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 a charger, whether some apps are running and consuming a lot of CPU, whether the display is on, etc.).

A rough estimate of the charging speed could be obtained after charging has been running for some time, but this isn't how people want to use the estimated time — they want to see the value at the moment they connect the cable to their device, not in the middle of charging.

I have tried several apps that try to estimate charge time, but none of them work reliably. Instead of providing wrong and unreliable estimates, I have decided not to implement this feature at all.