The sin of Korach (who didn't have a portion in the World to Come)... including a focus (and blame) on the wife of Korach and the wife of On (who names Korach and his followers as holy people, and then entices them). Plus, a discussion of the power of the Leviim and Kohanim, and the famous dispute over techelet on a garment. These interpretations are midrashim that build out the biblical text. Also, a new mishnah about the exile of the 10 Tribes, Rabbi Akiva uncharacteristically maintains that they will never return, and various others disagree with him as to their capacity for return.