Add pen sets
I understand that the ON team wants to keep the UI minimalist, so they've limited the maximum number pen slots to 15.
However, this simply isn't enough to capture all inking contexts (eg. different page colors, technical vs. notetaking vs. doodling, etc).
The solution, I think, is to offer "pen sets".
Create a drop down menu on the pen toolbar to allow switching the current pen loadout between user-customizable pen sets. This would allow us to quickly access different color themes ("pastel", "dark", "rainbow", etc.) and different pen types/thicknesses ("pencils", "inks", "thick/thin").