Agreed with all of this--especially the point about how people's priorities change as they progress through life. Supposedly we all go through a new life stage every seven years, so what the core audience values now, will likely shift over time.
WOTC has produced tons of splatbooks expanding player options, but not a lot of expansion support for DMs. This is probably because everybody buys the player options books, but more DMs than players will buy DMs Guides, resulting in less sales.
5th Edition is a pretty flexible game system. I can't see why WOTC can just put out a book of "Game Modes" for D&D, with tutorials on multiple approaches to play. Then, write up a chapter teaching how novice DMs could DIY their own setting, put together using templates provided in the book.
You could have a chapter on each genre or play style. This could serve to include your entire player base, encourage them to try out different kinds of play styles under the "D&D Umbrella", and capture all of the customers. This would also increase interaction between groups of players, and introduce cross pollination of play styles and bring disparate players together as groups tried out each others' games.
There could be chapters on short term campaigns, and running very long campaigns. You could also have an introductory rules set, intermediate and hard mode. The hard mode could increase player mortality, and up the challenge in general, for players who are less casual and enjoy beating challenges as a team.
Genre chapters could include: Gritty Swords & Sorcery play, Planetary Romance/Science Fantasy play, CW-style fantasy teen relationship play (which I think is what Strixhaven is), Epic Heroic Fantasy, Classic Dungeon crawls, Crit Role style comedy romp/set piece action sequence games, Mysteries (like Candlekeep), Historical Realism or Arthurian Romance.