While the main Diablo IV story will be focusing on Lilith, but each expansion pack may come with a different antagonist. Diablo could make an appearance in later expansion packs, and some expansions will feature never-before-seen antagonists. While this should keep progression fresh for an extended time, it is unclear if Blizzard intends to charge players more money by expansion.
https://www.notebookcheck.net/Blizzard-shares-more-info-on-Diablo-IV-expansion-packs.444837.0.html