No specific element is mandatory to improve engagement, but something on the pages has to do so, to optimize the user experience. If the page is to only have text, perhaps the font of the text body should be larger (like 125%) to encourage readability. The overall design or theme layout should be cleaner and nicer.
And the element that is added can be mixed up---images on some pages, an embedded video on other, a quiz or poll here and there, a download or giveaway, or some combination. But if the text body is tiny, ugly, and usually not paired with any factor to encourage dwell time, a higher bounce rate will follow.