Diablo general manager Rod Fergusson announced today at the DICE Summit that the second major expansion for Diablo IV will not be released for the action/RPG sequel until next year (thanks PureDiablo). IGN summarizes Fergusson's comments, which suggest the schedule for this got pushed back as the ripple effect of resources being shifted to live content updates during development of the first expansion. So the bad news is there will be no expansion on the 2025 roadmap, while the good news is there will actually be a 2025 roadmap:

Speaking today at the DICE Summit in Las Vegas, Fergusson concluded his talk by discussing the team's plan to strengthen its relationship with its community by taking a leaf out of the books of both Diablo Immortal and World of Warcraft and releasing content roadmaps. He said that soon, we should expect to see a content roadmap for Diablo 4's 2025 plans, outlining seasons and updates at roughly the times they would show up throughout the year.

However, he said that the game's next expansion wouldn't be on that roadmap. It's not coming until 2026.

"In 2025, or just before Season 8, we will have a 2025 roadmap for Diablo 4. Now, our second expansion won't be on that roadmap, because our second expansion is coming in 2026, but at least players will have the road ahead."