You want specifics. I said it before and I said it again and I'll say it one more time.
I want the book to be laid out and organized so actual people can actually read and use it. An actual functional index would be revelatory.
As an actual person I read through it and didn't have an issue understanding it. I think it could be organized a bit better and of course everything can be improved. Much of it feels ass-backwards, put Running the Game at the front. World building and the multiverse at the end with treasure being the last chapter.
I still don't get all the complaints about the index. People still use them? It's an index in an age of google.
I want a basic explanation of what people usually expect from the game as players. This might need some granularity and some acknowledgment that people sometimes want and expect different things. It should probably include some specific advice for meeting those expectations.
They do talk about it here and there explaining why some people would want some style and not another. You have read the DMG, right? There's talk about different styles of players in the introduction, for what it's worth.
I want some specific commentary on why a DM would run one way as opposed to another. This doesn't need to be judgmental but rather than just telling the new DM they can run in these ways it seems useful to explain at least some of why people run those ways.
The result would probably disappoint. Because it would typically come down to "We looked at older games and what worked and what didn't. Then we discussed it as a group and made the best decision we could. We adjusted some things based on play test feedback."
I want the effects of and reasoning behind optional rules explained. I'd strongly prefer for those optional rules to have actual thought put into them and I'd strongly prefer for the explanations I want to actually reflect the effects of the optional rules on play.
I'll go back to roll of the dice. They explain the pluses and minuses of choices. Roll to resolve uncertainty almost all the time?
Relying on dice also gives the players the sense that anything is possible. ... A drawback of this approach is that roleplaying can diminish if players feel that their die rolls, rather than their decisions and characterizations, always determine success.
Ignore the dice?This approach rewards creativity by encouraging players to look to the situation you’ve described for an answer, rather than looking to their character sheet or their character’s special abilities. A downside is that no DM is completely neutral. A DM might come to favor certain players or approaches, or even work against good ideas if they send the game in a direction he or she doesn’t like. This approach can also slow the game if the DM focuses on one “correct” action that the characters must describe to overcome an obstacle.
There are other examples and explanations throughout the book.
I want instruction on worldbuilding to focus on getting the most play out of the least work and getting quickly to play. It's fine to work out complicated geography and functional calendars but I do not believe those things are exactly necessary to start play. Telling new DMs to figure out some broad strokes of the setting then some specifics of where play starts then fill things in as needed would be nice. An example of doing so with thought processes explained would be superb.
They give advice on that. It does have a lot of options, but that's kind of the point of D&D that the feel of the world you create can vary significantly. If you want something handed to you, get a starter kit.
Is that specific enough?
All those things seem as though they might be more helpful for newer DMs and some might be useful for more experiences ones as well.
I know this is a broken record, but most DMs don't start in a vacuum. They have experience with D&D as a player, have plenty of background in fantasy worlds from varied sources including video games to novels.
Last, but not least, there is no such thing as perfect. A book that works for 80% of people is not going to work for 20%. As an IT person, there were many times when we had to focus on that 80% because that 20%? That was more effort than the rest of the 80%. The last 5% could be the most costly of all.
There is a trade-off. The core books are all between 320 - 350 pages. Obviously they have a page count target that they adhere to pretty strictly for a variety of reasons. There's only so much you can do with that limitation and the result will always targeted, the book is not targeted primarily at people truly new to D&D. That's what the free PDFs, encounters and videos along with the starter sets are for.