I often teach from PDFs that I want to slowly reveal small portions thereof as I lecture. The new "elements" feature whereby you can insert an image (like a blank sticky note) and set it on top of text/information/answer is a step in the right direction. If I could tap on the element and it would become invisible (but not erased), that would allow me to slowly reveal what I am presenting in lecture as I lecture. In addition, when I close that PDF after lecturing, all elements would be reset to visible.

I think the ability to tap on an element to make it visible or invisible (without erasing it) would be a great study tool/feature for students as well. Essentially, it would serve as a "flash card system."