In entirely unsurprising news, Blizzard has officially confirmed that the upcoming ARPG Diablo 4 will be monetized. While Diablo 4 won't be revisiting the controversial days of Diablo 3's auction house, or anything else of that severity, it does sound like Diablo 4 will be adopting some modern-day monetization practices. Blizzard states that it's still very early and that monetization plans are in flux, but does make clear that Diablo 4 will not sell power.
Twitch streamer Quin69 got the question answered at BlizzCon 2019, speaking with Diablo 4 lead game designer Joe Shely. Shely confirmed Blizzard's product plans in a simple statement. Diablo 4 will have a purchasable base game like previous Diablo releases. It will also have expansions, also akin to all previous Diablo releases. And finally, Shely confirms that, "You also will be able to acquire cosmetics in the game." Given the context of this sentence, "acquire" can be assumed to imply "purchase."
Quin69 asks a followup question, questioning whether Shely is specifically referring to in-game microtransactions. Shely says that while Diablo 4 is still early in development, microtransactions would be accurate. Shely later characterizes this by saying that microtransactions are "an opportunity to have more options available than you would otherwise." Given the modern popularity of cosmetic purchases, it's hard to argue that they wouldn't be popular even if the Diablo franchise has never featured them before.
Diablo 4 could potentially employ any of a wide range of microtransaction scheme and they'd work well with the game. Plain microtransactions that are bought and sold would work, but are uncommon anymore. A battle pass would work well in Diablo 4, given its seasonal structure akin to Diablo 3. They could even build in some unique challenges like in Fortnite. And while it may not sound appealing, Diablo 4 could even work out a loot box system. It wouldn't have to be as overt as Overwatch's loot boxes. It could instead just be actual chests found in a dungeon, opened with premium keys. But let's not give Blizzard any ideas.
It's unclear whether or not the decision to add microtransactions to Diablo 4 will be met with controversy or not. On one hand, the Diablo franchise has never had microtransactions before. And the one comparable system, the real money auction house, was a disaster. But in 2019 it's rare to nonexistent for an online game not to feature cosmetic microtransactions. At the very least, any vocal frustrations will likely wait until development moves further along and Blizzard provides more specific details about Diablo 4's monetization.
Diablo 4 has no current release window but is confirmed for the PC, PS4, and Xbox One platforms.