I think that proper layouts for the keyboard are good, but I think that would be better if the keyboard had the possibility to modify those layouts and make a custom one. I believe that could make this keyboard unique and the best one, because every user could make its own keyboard.
For example: I'm a Spanish language user (sorry if I made mistakes writing this) and I don't like to have the "ñ" letter on the layout; it only takes space and it's not used very often. But if I choose the English layout, so the "ñ" doesn't show any more, now I can't access "¡" and "¿" easily, I know that I can add them to the prediction bar, and so I did, but I have to accept a word for the symbols to be displayed. So I think that it would be far more convenient if I just could decide what letters a certain pop up shows, in that way I could make that the popup for the letter "m" show "?" and "¿", and even define which is first (default).
This feature could also be useful to remove characters that are never used, i.e. in the popup for the letter "e", I could remove "è", "ê", "ë", "ę", "ě"; so I could access "€" faster and easier.
And so on, the possibilities are endless, even the arrow keys could be added as a pop up key, or have the popups for the numbers rearranged, etc.
I really hope that you consider my idea and find it useful, thank you for your time.