When editing a document that is of slide mode, the preview (in split view) live updates as expected. Now it would be kind of elegant if this were also to be the case for the slide that is open at the same time in presenter mode.
You could change your slide as you give a talk.
Often, in presentations, live editing is “too far away” to bother. Hedgedoc is already a good step in the right direction – maybe this could make it more complete.