Please allow user to place characters in place for long press on several keys.
This has been suggested for Android (by Matt Winans).
Customization is important because there are many special characters, and there is large variability in the characters to which different users wish quick access.
The concept appears in Blink keyboard, but their implementation is terrible.
It’s used by quite a few people in French-speaking countries.
It’s already present in AnySoftKeyboard, Swiftkey on Android, Multiling O Keyboard.