I think that we might be a little biased because we know the challenge is to create a next button and thus, we're going to be on the lookout for it. I also think that our ability to quickly identify the next button is due to our experience with e-learning and having seen many examples of course navigation. This is definitely not the case for all audiences and I find that I usually err on the side of making things explicit, just in case. However, I thought that the images you used in the first and second examples (door handle, arrow) and way you used animation and bright text to catch the viewer's attention in the third example worked well here to prompt the viewer to interact with those pieces.
Like everyone else, I didn't even know the timer was there and stayed on each slide for under ten seconds. I love the idea behind it and how the animations were subtle enough to catch your eye.
I would say that the amount of time on the timer should be dependent on the amount of content on the slide. I usually time how long it takes for me to process the slide content to approximate the timing of animations like these. I thought that 10 seconds for each slide was appropriate for your demo, Jonathan.
This was a great question and thanks for prompting an interesting discussion! For my demo this week, I tried to use subtle animations for my buttons but ended up labeling them for clarity because I didn't think they were intuitive enough. I'm considering going back and seeing if I can improve my demo. (Maybe adding a brief timer before the labels pop up!)