Realistically, a playable D4 server emulator is unless Blizzard releases official offline support (unlikely). The effort is comparable to early WoW emulation – expect 2027–2028 before you can play through Act 1 without major bugs.
: Diablo 4 uses advanced encryption to protect its data. Emulators require a "hook" or a custom launcher to bypass or decrypt these packets so the emulator can read and respond to them. diablo 4 server emulator work