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
If this is push by feminism, it somehow feels much more refreshing having female main antagonists than main protagonists, as the later often comes off as cliché and "trying too hard", especially when the role seems like a male character with a female cast(similar to forcing a black Snow White on people, just create your original African classic and leave European stories alone).
"Hopefully Blizzard finds a way to keep all expansions free"
HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA