My opinion is Planescape will return before Dark Sun. It is not only a lot of work with the psionic powers but DS has got a special tribalpunk style and this means work for the art designers. My mind is open about a graphic novel publishe by WotC itself set in Dark-Sun.
There is an opened door for Birthright, but not in this phase yet, but we will have to await more time. Hasbro is very interested into the digital market, and not only videogames about dungeons-crawling but also strategy. Birthright should be the best option to start from almos zero.
My theory is Hasbro wants D&D to be as Fortnite, the king of the intercompany crossovers, and then the target of 6th Ed is to be a system for all the genres: fantasy, space opera, superheroes, steampunk, gothic horror.. to make money with licenced titles, (videogames, comics, movies..).
Can I say "otaku" or is this too offensive? I guess Hasbro is interested in the potential market of manga+manhwua+manhua, but the steps should be careful to avoid complains about cultural appropiation or something like this. Have you seen Kamiwaga: Neon Dinasty? Even a manga in the web.
Mystara is too generic fantasy, but there is potential in Hollow World and Red Steel.
Witchlight could be a setting. Don't you remember Ravenloft started as a module?