AgreedHere's what I'd like.
- Move magic items to the PHB. I consider that player-facing anyway (especially when their characters have the ability to craft magic items.)
Quibble, here something along the scripted example of play from the Old D&D Basic Red Box would be good but I think that should be a free online document, referenced in the DMG and the SRD. That should discuss, player agendas, and different campaign styles and how one may implement them and resources on that topic.
- Drop the high-minded "how to be a DM" advice. There are better examples of actual plays and DM advice online.
OK.
- List many examples of traps (including complex ones) and environmental hazards
- Have a completely overhauled "player rewards" section. How to award XP, treasure, etc.
Absolutely
- Downtime rules
OK, I would add they should be fairly explicit on their encounter assumption and what to do it you find it too easy for your party. Upping the CR vs more lower level monsters.
- Encounter design
This should be a big section because there are a number of incompatible popular approaches to the topic of travel and exploration.
- Travel vignettes, skill challenges
I think a detailed review of the old D&D resource management hex crawl is in order and then go on to more relaxed version where the encounter pattern is followed but without the resource management and how it breaks down around levels 7 to 10, due to assumed magic (items) and how things might be adjusted.
Then discuss the journey and skill challenge approaches to wilderness exploration.
An Extended discussion of different world building assumptions, gritty and/or low magic, level caps below level 20 and so forth.But basically, I think we keep getting DMs Guides because of tradition. I don't think it's a necessary book. I'd like to see a two-core book model.
Treasure, crafting, downtime rules in the PHB.
Traps, encounter design, hazards, skill challenges, etc., in the MM (adversary guide?)
Have the DM's advice stuff on D&D Beyond columns and in-print in the various "beginner DM" adventures.