Thanks @ZinggJM for your fast feedback and the link, I will have a look into that!
Hope no offence taken :). My request was not for to ask for justification, more on the thoughts process behind so find out how I would extend the library in the best possible way, so others might be able to profit from it too.
I'm currently extending some of the classes and will let you know if there is something valuable coming out of the exercise. I'll publish it at michaelschnyder/GxEPD2 at Add-Support-For-Custom-SPI-Pins (github.com)