An Unearthed Arcana 5 Ed with optional rules I have suggested in other posts.
A new Savage Species with the return of monster classes. All the dragons, not only metalic and cromatic, as monster classes.
Dreamscarred Press being hired as outsourcers for a new Tome of Battle: Book of Nine Swords and Magic of Incarnum
A new Chronomancer sourcebook introducing alternate timelines and parallel worlds, and a demiplane created by the collective memory from the past, to allow visitors "isekai" from people who read the novels and now they are within these.
In a comic a crossover Ravenloft/Ghostbusters, or Ravenloft/Buffy vampire slayer.
A Ravenloft spin-off about a (noir-punk) world (from the material plane) ruled by secret societies and lodges of supernatural creatures, for example vampires.
A new PC race,
ipotanes, antropomorphic horses/ponies with the design of the head enough realistic for an action-live production, but also enough cute to be sold as toys for children.
A D&D mash-up version of superheroes franchises, when nerfed characters if it was necessasry for gameplay and power balance.
Pantheons of the preislamic politeism in Middle Orient (Sumerian, Babilonian....)
The aliens races from d20 Future in Spelljammer.
The inhumanoids as DS&D monsters with stats in the future April's Fool.
A strategy videogame based in the setting Birthright.
A survival sandbox videogame as Conan Exiles set in Dark Sun, allowing a crossover with Jackandor.