Here'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.)
Magic items belong in the DMG just as monsters belong in the monster manual. The PHB would otherwise just get too unwieldy. Of course this is all a print problem. I almost entirely use DnD Beyond to look up magic items using filterable list section it has for the purpose. Personally, if I were relying on print, I would want all the magic items from the various rule and adventure books printed in a separate book on magic items. I would also prefer spells to be in a separate book. I would just find that must more convenient to use in game. Even when I was running games in person, we used spell cards. I realize I'm in the minority here, which is fine. As long as I have D&D beyond, it doesn't really matter to me. But I don't think I've look up a spell or magic item in the PHB or DMG in over 5 years.
- Drop the high-minded "how to be a DM" advice. There are better examples of actual plays and DM advice online.
The print books are for people who want to engage the rules in print. If you are going to design the book on the assumption that people should consult online sources, why have print books? The DM guide should, in my opinion double down the how to DM. The basics of how to run a game should be in the PHP, but the DMG should give advice, examples, and tools to help you improve your DM skills and run various styles of games.
- List many examples of traps (including complex ones) and environmental hazards
Yes. 100% agree.
- Have a completely overhauled "player rewards" section. How to award XP, treasure, etc.
Agree. The default XP rules plus perhaps a short treatment of simple milestone leveling should be covered in the PHP, but the DMG should do a deeper dive into different approaches. XP for non-combat, mini-milestone approaches, etc.
Yeah. There is some good stuff for downtime, but it is currently spread accress at least three books: PHP, DMG, and Xanathars. I would like to see downtime rules consolidated and expanded. Included in this could more fleshed out options for stronghold rules, faction & reputation rules, and expanding on the Xanathar complication tables, which I find to be a bit uninspired.
This is one area I am confident they are going to redo. I would like to see encounter design to include mechanics for adding environmental challenges into your encounters.
- Travel vignettes, skill challenges
Yes please! I would also like more ideas on how to make passive abilities more flavorful. More tips on how to balance dungeon crawling/exploration between a constant roll to check slog on the one hand and auto-pilot passive checks on the other. I think skill challenges has a place in this.
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.
Personally, DnD Beyond has me past the book model. I would love to have DND Beyond to continue organizing all of its content by major categories. I should never have to select a "source". But for those who like to work with the print books, I think you would end up with huge, unwieldy tome that would intimidate new players, and annoy players who want a way to quickly reference a rule in print. For print, I would prefer going the opposite direction: Put spells and magic items in separate books. Have PHP, DMG, MM, Spell Book (SB), and a Treasure Chest (magic items, expanded treasure and equipment items, tables, and rules - crafting rules could go in here as well)
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.