I used to write with a fountain pen all the time but switched to iPad because I had so many notebooks it was getting a bit ridiculous trying to go back through my notes. One thing I miss so much is the way ink looks on a page, particularly inks with hi sheen and shading. Colours vary in intensity at different points on the page, some completely change depending on the amount of ink laid down and it really makes writing enjoyable and makes revising notes oddly satisfying. Please add some form of this to GoodNotes as an option when selecting colours, It would make the writing experience amazing and would work so well with the fountain pen tool.

This article has a great range of these effects;

https://mountainofink.com/blog/sheen-sheen-sheen