Fifth Edition Fantasy #14: Beneath the Keep - likely originally intended to be included in OAR1.OAR 5 Castle Amber - More details here (Sept 2020).Isle of Dread Walkthrough Map (Isle of Dread).D&D Next Playtest Files October 2012 (Isle of Dread).D&D Next Playtest Files September 2012 (Caves of Chaos).D&D 5th Edition Basic Rules (PDF) - Contains several Mystara name checks.
Ghosts of Saltmarsh - Specific mention of Mystara and several Mystara locations.Baldur's Gate: Descent to Avernus - Tortle character named Krull.See You There In the Nine Hells! - Cartoon inspired by Descent into Avernus, by Jason Thompson, features a Cleric of Ixion.Rime of the Frost Maiden - includes Magen, Frost Salamanders.UA's Races of the Feywild introduces new races, including the Hsiaoo-like Owlfolk.
The Wild Beyond the Witchlight: A Feywild adventure (2021) - said to include Warduke, Strongheart and Kelek from the LJN Series as well as Giant Snails, Redcaps and a few other creatures.WotC 5E/D&D Next Books with Mystara Teasers Mystara Characters in Non-Mystara Books, AD&D Mystara Product List, 3E/4E/5E Mystara Product List, LJN Character Product List, DragonStrike Product List Atlas Rules Resources Adventures Stories FAQ Search Links 3E/4E/5E Mystara Product List (and Downloads) by HÃ¥vard