What Is One Small Course Design Change That Improved Learner Engagement?

I can't speak too much to completion rates since our learners more or less have to complete content. I'm sure there are a handful of people who start optional content and don't finish it for a few different reasons, but we don't run reports on that since the reasons could be myriad.

If by engaging you mean interesting enough for people to maintain relatively constant attention throughout, I try to leverage multimedia theory. A slide filled with text that a narrator reads to a learner is a ticket to snooze-town. Images instead of words can communicate more. Animation that only reveals words once they're relevant can reduce the kind of cognitive load that undermines learning rather than supports it.

If by engaging you mean content that requires or begs interactivity, I try to remember to add in learning, not labor. Click-to-reveal interactions in many ways insult our learners. Where possible, I try to have interactions be "procedural representations" of processes and ideas directly relevant to the content. I once took an eLearning about HIPAA compliance that had a quiz, and the more questions you got right on the quiz, the more snowballs you were awarded to then throw at a yeti after the quiz... Why? Interaction should be meaningful.