One of the BEST things I've found at improving CTR is using polls/trivia.
It gameifys the newsletter and encourages interaction.
Also, asking real, provoking questions that solicit responses. At my day job (soon to be my former job because I'm being laid off 😢 ) I've had a lot of success with getting our newsletter editors to talk to their audiences and ask them questions. And even asking readers to respond via a simple form/survey works.