Remember that SKT was the most errataed AP book recently. Errata to include the update to the new format for spellcasting monsters, no less.
Maybe a deluxe reprint of SKT is in order, remembering that for most people that book is THE northwest Faerun gazzetter of 5e.
Also, the Deck of Many Things... maybe Gremore Abbey will get a "CoS" treatment becoming a full 1-8 adventure?
I have got other idea. A sequel of the module of "Black Spine". This time it wouldn't be PCs from Athas towards the bad guys' lair, but PCs from the D&D multiverse catched and sent to the secret lair's, and maybe an open door for Dark Sun, or a spin-off in a "wildspace" next to this. Maybe the faction of the Black Spine could discover and explored the world of Jackandor. This could allow a mash-up gettings elements from Athas and other sources, and with an unlocking in DM Guild third parties could publish a lot of new ideas, or art imititating that primal-punk style.
I wonder if WotC may be interested in a hybrid of d20 and Magic: the Gathering, recyling some ideas of the cancelled MMO of Magic the Gathering. The idea would be the players collecting the "cards" as once-daily magic item.
It seems like they could do something Ysgard-related: it has got giants and is planar (and I am pretty sure there is a portal to Sigil somewhere on Ysgard). They could throw in some Viking-related player features (new toys for the martials), maybe even find a real Scandawhovian to make sure it is culturally appropriate....
Ysgard also has the advantage of being easily self-contained (you don't need hordes of demons when you have hordes of giants for example).