Thank you @algernon for all the great ideas and effort you are already putting into the extension libraries. Since you asked: I am looking for a library to implement a “diacritics” key.
The idea is, that instead of typing a dead key followed by the letter you want the diacritic to appear you first type the letter followed by a fixed “diacritics” key that then steps through different diacritics the more often you type it.
For example you would press an ‘e’ followed by the diacritics key which would delete the e and type an é. If you would press it one more time it would delete the é and type an è and so on. Depending on the language you would have different maps for the diacritics key.
In German you would for example have o -> ö -> ô -> ó -> ò in French you would rather have o -> ó -> ò -> ô -> ö
The idea origins from ADNW - Optimierung Für Die Gerade Tastatur Mit Daumen-Shift.