Dragonlance is one of the favorites, if it is not in this "phase" will have to be the next. It is the best option to sell merchandising products. Here the idea of a reboot is very risky, because there are dozens of novels that fandom wouldn't like to be decanoniced. An option would be "this is an alternate timeline", but this concept could ater the cosmology of ther rest of D&D multiverse.
The 3.5 Ed origin of the dragonborns was ordinary humans and humanoids chose by Bahamut to be his champions, and after a ritual and a "crysalid" phase transformed in dragonborns.
Mmmm, then maybe the origin of the kobolds is a goblinoid race chose to be Tiamat minions.
We could say FR ate almost all the D&D multiverse. This doesn't matter too much. The races and factions from Spelljamers could explore the Astral Plane.
Mystara is practically
"shut up in the chest of the oblivion" ( = totally forgotten) but it can offers two or three things: A new partnership with Capcom for an arcade videogame. Savage Coast/Red Steel (pirates and mutants, for the audience who love Jack Sparrow and the Caribean Pirates) and Hollow World (ancient cultures, dinosaurs and prehistoric faun). Possible in a far future, but not soon.
Kara-Tur and al-Qadim are possible, but today they need a lot of work to be politically correct, and this is not fandom's fault but by other collectives.
Hasbro should negotiate with Gygax's heirs about the rights for Gord the rogue novels and Dangerous Journeys, because if the D&D action-live movie is a blockbuster, then others will try to buy the rights for an adaptation of the franchises by the same author. WotC could publish a sourcebook about the word of Mysta with an agnostic system, only background, and allowing this being used for webnovels.
Other suggestion is to allow "isekai" fiction, for example a secondary character of "Jem and the Holograms" is sent to Dragonlance. Really she is not in the original Krynn, but a "clone demiplane", something like the "dream land" in the novel "Tanis: the shadow years".
Ghostwalk could return as a softer spin-off of Ravenloft, and new plane for Magic: the Gathering.
Councyl of Wyrms could be rebooted as Chris Perkin's Iomandra (homebred setting), but the 5th Ed is not ready yet for dragon PCs. I advice await until almost all the dragons from previous editions to be updated. Dragons are perfect to be sold as toys for kaiju-fan children.