But the remaining page count could be just fluff; Xanathar rambling on, or something similar.
Based on previous books, we should be lucky to get a hundred pages of solid crunch.
I mean, we don't even know if we're getting the Revised Ranger, or any feats. And nothing says we'll get high level spells - we might just get exactly what we've seen in UA, and that means "starter" spells.
(There was one thing you didn't mention: and that is new rules for magic items, whatever that means. I mean, I dare not hope for a 3e-style utility based magic component pricing system...)
There are sooo many things the game is completely ripe for. Instead of repeating all of it, I'm just going to say "Advanced Dungeons & Dragons" and refer to those threads.