Any progress made on a hypothetical offline mod would never be transferable to the official live servers. Blizzard’s Stance: Why No Offline Mode?
"Offline Mode Mod for Diablo 4: A Game-Changer for Fans of the Series?" diablo 4 offline mod
Unlike its predecessor, Diablo 2, which stored character data locally, Diablo 4 handles almost all logic on Blizzard’s servers. This includes: Loot drop calculations. Damage and physics interactions. Character progression and inventory. Any progress made on a hypothetical offline mod
While an official offline mode is unlikely to be patched in by Blizzard, history suggests that the community eventually finds a way. It took years for stable private servers to appear for games like World of Warcraft or Diablo 3 . which stored character data locally