I know a lot of people have theorized a new Forgotten Realms book at 1D&D's start, but with all the changes made to race (and subrace) design, as well as with Feats, I think Eberron is going to desperately need an update moreso than Realms is.
Most of Eberron's races got an update in MotM, with only Warforged and Kalashtar and both of those work fine with One D&D. Artificer is likely going to become a Core class.
The only issue would be the Dragonmark subraces.
But FR on the other hand needs a new book far more. It has races like Wemics that haven't been touched yet, it has everything outside the Swordcoast barely explored at all, to the constant complaints of FR fans for years. FR never really got a proper book that even explored it's main continent properly.
And there are so many changes to races happen like Tiefling, this Ardling thing, Aevendrow, Lorendrow, and so on. It's also an opportunity to sell the book to new FR fans generated by Honor Among Thievey if it's a success.
Plus I expect that the Forgotten Realms will be the setting that gets a connection to the MtG multiverse thanks to Realmbreaker. If so they could use FR update some MtG races.
I think that's one of the purposes of the Phyrexian arc in MtG next year, isn't just to undo part of the mending and link various MtG planes together, I believe they plane to use Realmbreaker to also create a link to the Forgotten Realms beyond the blind eternities.
They've been building up to this for years now, D&D races in Strixhaven, a Ravnica FR Crossover campaign.
On a practical level building a bridge between FR and MtG via Realmbreaker makes it way, way easier to reprint D&D cards in MtG sets and vis versa.
So there are a ton of lore and mechanical reasons to return to FR, plus a lot of folks were unsatisfied and even unhappy with the SCAG.
Also I think the MtG sets in late 2023-2024 rotation all have D&D vibes to explore the new connection. Eldraine might get a Elminister Planeswalker and Ixalan maybe Elliwick Trumblestrum Planeswalker for example.
An FR Campaign Book could explore the effect on FR, like Phyrexians invasion.