It was revealed during D&D Celebration 2021 that a third classic Dungeons & Dragons campaign setting will be releasing in 2023, as two others are already set to launch in 2022. The current edition of D&D uses the Forgotten Realms as its default campaign setting, but there are books available that help groups to use other worlds for their games.
The most recent classic campaign setting to return to Dungeons & Dragons was Ravenloft as part of Van Richten's Guide to Ravenloft. D&D's fifth edition has not only brought back classic campaign settings, like Eberron, but it has also integrated planes from Magic: The Gathering into the D&D multiverse, including Ravnica, Theros, and - soon - Strixhaven, which will be introduced in the upcoming MTG crossover Strixhaven: A Curriculum of Chaos.
Wizards of the Coast had previously confirmed that two classic campaign settings were in development. These campaign settings are due to be released in 2022. Wizards of the Coast revealed during the Future of D&D panel at D&D Celebration 2021 (which can be seen on the official Dungeons & Dragons YouTube channel) that a third campaign setting will be hinted at in 2022, with a full release happening in 2023.
It was also revealed that the two returning Dungeons & Dragons campaign settings in 2022 will be published in an unconventional format, suggesting they won't be standard rulebooks. The third campaign setting was mentioned in relation to the D&D multiverse, which means there are two strong contenders for what it will be: Planescape or Spellhjammer. Planescape involved the multiverse as a whole, including the spaces and cities between worlds, so it would be a natural fit for tying the multiverse together. Meanwhile, Spelljammer is a highly-requested campaign setting, involving magical spaceships that can travel between the planes.
The fact that there are three campaign settings returning could mean Planescape or Spelljammer are a natural fit for one of the slots, as they help tie the franchise's current direction together. The Wild Beyond the Witchlight campaign recently introduced a planeswalker from Magic: The Gathering as a character in the story, further cementing the idea that the D&D multiverse will be expanding. If Dungeons & Dragons and Magic: The Gathering are going to be further integrated, then it makes sense for the barriers between their worlds to loosen. From a lore perspective, this would make it more logical for characters to be able to travel between franchises, whether it be through magical spells or behind the helm of a Spelljammer.
Two classic Dungeons & Dragons campaign settings are coming back in 2022, and a third one is returning in 2023.
Source: Dungeons & Dragons/YouTube