“We are not going to support an offline mode,. But as I said before, nothing in Diablo 4 is going to require partying up. You can play solo and dungeons are private. Campaign quest areas will [also] remain private.”

As above is confirmed by Blizzard, please add an offline modus. There is nothing against online co op or special online events but the base game should be offline playable. One of the reasons is unstable internet will cause a real bad experience in the gameplay.

For example in the past 2 years my own internet connection has been interrupted months at a time due to digging through glas fiber cables in the ground.