"Next" Button that Pops Up

Here's a suggestion. Create a trigger that hides the Next button on timeline start, and a second trigger the shows the Next button when a variable reaches a certain count/value. Triggers could be added to each video module that increases the VideosWatched variable by one at the end of each video. I would be surprised if these forums don't have examples of this trigger and variable combination, discoverable using the forum's search feature

You could also use your 'watched' objects that currently appear when they've visited each video layer. Create a tigger that says something like 'show next button when the state of all the watched objects are normal' (or whatever the state is that they are in when they are all on).