I'm an architect, use Goodnote for markups of architectural drawings in the field so I like the simplicity of the pen selection - don't make it too fancy. That said I'd like a custom color option and I'd like to select both color and thickness in one shot without having to open the dialog twice.

Also ability to add custom colors to highlighter function.

I agree, a color mixing well would be ideal. Also, more pen size/styles is exciting but please put a button in the zoom window to bring up those controls so we don't have to go all the way top screen to access. I do most of my writing from the zoom window.

It would be great to change the top menu icon when changing the pen style as to know which one is active.

Also it would be great that the line style/thickness is a bit better represented in the icon.

Using the app a bit more I realise that the current fountain pen profile corresponds rather to a rollerball pen and the current rollerball pen corresponds to a ballpoint pen. It would be great if you could work a little bit more on the fountain pen realism but with my comment you could eventually have a third profile: the ballpoint which I think is quite useful.

It would be nice/fun to make more realistic the fountain pen ink. Not only the strokes are larger when the pen moves slowly but the ink tend to be darker than when the pen moves faster.

Also the main colors (Black/Night Blue/Red/Green) are quite realistic it would be good to add colors such as a lighter blue, violet. It might be good to think about having different sets of colors between the fountain pen and the rollerball pen.

And sizes.

Perhaps add a sharpie type thickness too. It'd make it easier to black things out.