I would love to see this feature, too. I review a lot of PDF's on my Ipad and instead of having to ask the person who created it to make amendments, a white highlighter option would act as sort of a digital white-out so I could make edits myself.

A white opaque option would allow one to achieve any color variation. In goodNotes you can overlay darker and darker, but there is know way to go from dark to light. If the white on the highlighter tool acted in this fashion then it would double as a paint brush. By the way, white at the moment does not do anything in highlighter mode. This should be addressed. I am guessing it is to make files less bogged down with high graphics, but even so this is a necessary feature if you want to go back into files and highlight areas that already have marks on them while avoiding covering lines.