It would be very interesting to be able to add images and dynamic interfaces onto the slide while in presentation mode via PowerPoint web add-in.
This would make the presentation interactive and allow for a lot of activities for the participants. As of today, you cannot edit the current slide if you are presenting it in fullscreen/slideshow mode.
That way, if you have an external platform for live polling, you can show how many participants have answered so far, you can show the notifications from Q/A on the slide, and you can let participants post images/text to your current slide.
