It seems that the merchandise people just couldn't wait for an official reveal, as they decided to upload posters and t-shirts on the Blizzcon gear website for an upcoming Diablo project called Diablo: Reign of Terror.
Blizzard likely wanted to announce the next big Diablo project during their upcoming Blizzcon convention, where the game could be shown at it's best with an awesome trailer that excites fans and intrigues those who may be tempted to try the series out for the first time.
The Diablo: Reign of Terror merchandise has given no clues as to the nature of the project. It's unlikely that it's an expansion for Diablo III, as there is no three in the title, with the same logic being used to argue against it being Diablo IV.
The existing Diablo: Reign of Terror merchandise only shows a picture of Diablo himself, which doesn't reveal much as he's the main villain of the series.
It's possible that Diablo: Reign of Terror is a new game in the series that isn't a direct sequel to the events of Diablo III and its expansion. The original Diablo is a likely contender for a remake, considering how long it has been since it was released.
The Diablo series has expanded its audience over the past few years with Diablo III being released on consoles, with a port of the game coming to the Nintendo Switch in the near future. It's possible that Blizzard may want to capitalize on this new audience and bring them up to speed on the history of the series, which may even lead to an eventual remake of Diablo II.
There have also been rumors of a Diablo animated series being produced by Netflix, which would be a strong candidate for the identity of Reign of Terror. Netflix had a lot of success with their popular Castlevania animated show and may be looking to expand their range of video game adaptations, especially as they are working on a live-action TV show based on The Witcher.
The truth behind Diablo: Reign of Terror will likely be revealed at the Diablo panel at Blizzcon that is being held on November 2nd.