This idea is taken from other existing apps like Documents for iOS. When using the highlighter with text the colored line snaps to text automatically, in this way you have not to worry about going outside the line.

See the GIF to understand what I mean or try the Documents app

I would maintain the awesome straight line highlighting feature of GoodNote but I would add a new button for this new one or maybe as an option of the actual highlighter.

Thanks a lot and if you like this feature vote it!