I have read Slaughterhouse Five, The Sirens of Titan, Player Piano, and Cat's Cradle, so it is time to read Breakfast of Champions and Mother's Night.
Breakfast of Champions - Absurd humor with fun tangents and sketches that made me giggle outloud. The inclusion of the author into his story makes for good fun too. Several characters from past and future novels make appearances. I find it scary how many ills of society and nature are addressed here by Vonnegut, forty years ago, which are still prevalent, and more urgent, today.
Mother Night - Maybe my second favorite book, behind Slaughterhouse Five. Vonnegut has Howard W. Campbell, Jr. narrates his life story, by writing his memoirs (metafiction) as he sits in an Israeli prison awaiting trial for war crimes during WWII. Moral of the story - "we are what we pretend to be" and so on. Mr. Campbell eventually punishes himself for crimes against himself, and not the crimes against humanity.
Many themes/ideas/consequences to explore - Death, Nazism, nationalism, propaganda, eternal love, racism, artist's pursuit, self-identity, espionage, etc.