Sword of Spirit
Legend
Unless you started playing with OD&D, every edition had concessions to new players. Heck, the Red Box was a product solely and explicitly aimed at new players.
You know, that's a really important bit of data for marketing purposes. The Red Box is one of the most iconic D&D products ever, and introduced a lot of people to the game. That's definitely something to consider in any analysis of what works or doesn't work. (I'm pretty sure they've done that analysis, hence the Starter Set. It would be interesting to find out how well the Starter Set has been selling.)
I would probably do it in a way to really distinguish it from the rest of the line maybe with different binding and a lot of art and maps, but thick with a lot reference information of nations, organizations, personages and history, everything anyone could possibly ask for in a FR guide except for game mechanics and stats. Have it serve basically as a setting bible and and as a art/coffee table style book with a higher price point to make up for the fact that it is a niche product and outside the normal cycle of releases with player content.
This is good. I want that.
The 100 year leap was a critical error. I feel the same way about that when they did it with Star Trek: TNG from TOS. It limits storytelling more than it expands it, but that is all water under the bridge at this point. You can't reset without breaking everything else. Although I understand why creators do it. It almost never ends well.
What they can (and I think should) do, is cover multiple eras in the book. For instance, when covering Waterdeep, they give you the history up to 1358, then talk about the important people and events then, and then continue the history, pausing for more snapshots of the city at important points (I'd recommend that they coincide with the first presentation of each edition), and after giving a snapshot of the current 5e stuff, they give future possibilities, maybe three or four different paths that Waterdeep could take. And they do that for all of the other regions of course.
I think they could easily fit all this into a book without crunch, and considering the very concise prose of 5e products.