It could be as many as 320-pages, like Eberron but it could also be as small as Ravnica and 256-pages
A Wildemount-comparable 304-pages seems a compromise.
Races. Mul, thri-kreen, and goliath/ half-giant. This needs a couple pages on each of those as well as two or three pages on the classical races: elves, dwarves, halfings, humans, and dragonborn. Really, because each race is heavily revised, this will be a larger section than other books.
20-pages
Classes. Psion needs to be there. It encourage players uninterested in Dark Sun to purchase. This and spells will likely be longer than the artificer. And a few psionic feats should also be reprinted.
15-pages
Dark Sun Characters. Defiling and preserving. Elemental clerics, templar warlock. This could be expanded to describe how classes like the bard differ. Rules for obsidian and stone weapons.
10-pages
Backgrounds. Quite a few that can fit here. Maybe a revised gladiator, a slave, dune trader, apprentice templar, veiled alliance, etc.
6-pages
Gazeteer. This will likely be as long as Eberron or Ravnica got.
20-pages. If even that.
Adventures in Athas. Factions, some locations, and desert survival. Hazards of the wastelands.
30-pages.
Tyr Descripton. A description of this city-state and set-up for the fall of its sorcerer-king, like the 4e version.
15-pages
Adventure. The usual
12-pages.
At 128-pages this is potentially a third of the book, easily. If not more with extra art and maps. So a subtotal of 140-pages.
Ravnica had 79 monsters and Eberron had 30. I imagine this would have far more. The 4e Dark Sun Creature Catalog has 55 plus 15 NPCs. And I'm sure there are a variety of others and doubles, along with some big HIGH CR monsters that will likely take-up a couple pages.
90-pages seems reasonable, for a new subtotal of 230-pages.
So there's a little room for expansion and some extra content.
However, the book also needs some other new addition.
Ravnica had the expanded faction rules, Eberron had group patrons, Wildemount has the heroic chronicles, Theros has the supernatural gifts. There needs to be some new mechanical option or rules edition that can be adapted for other campaigns (beyond obsidian weapons and defiling).