Dear Diablo 4 team:
I have questions about the three editions of the game and their upcoming expansions. Will there be an expansion for each edition or just one that applies to all three editions? Should I buy the deluxe edition to insure there will be an expansion for it? If there are expansions for each edition, will they differ in features across editions?
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To answer your question though, for ALL Blizzard games.
There is one game. There is Diablo 4. That is what everyone buys and downloads. When there is an expansion that base Diablo 4 will be upgraded to the expansion if you purchase it.
The Deluxe or other editions are not different versions of the game. They are the same version with cosmetic or other added extras. They may or may not offer versions with cosmetic extras when an expansion is sold.
Regardless of what version of Diablo 4 you got, you can get any version of an expansion and it works with it.
NOTE: That applies to the platform you bought it on. PC on PC, Xbox on Xbox. You have to buy the game, and expansion, separately for each platform.