What should be put in the final slide when preparing teaching presentations?
Take a cue from TV. Pretty much every serial show I watch ends with "next time on..." It is simple, not prone to misunderstandings like trying to be funny, and actually serves a purpose of indicating what the students might look at before they come in for the next lecture. This is how I wrap up my lectures even though I do not use slides in the classroom. (I like to juggle multiple columns of coexisting text on a blackboard instead.)