The key event that is missing in the timeline for the New Kingdom (1550–1069 BCE) is likely related to the "Hittites and Mitannians in Palestine." This is because during the New Kingdom, Egypt engaged in significant military campaigns and conflicts with these groups, particularly during the reign of Pharaohs such as Ramses II.
Therefore, the correct response to complete the timeline would be Hittites and Mitannians in Palestine.