I agree with just about all the OP's suggestions, but I'll get into some specifics:
Things to add
- Social contract: guidance on setting up the "social contract" of the game, including session 0 checklist and advice on managing player interactions and expectations (the Level Up dmg seems to do this, but I have not read it)
Party of "what is an rpg?", "basics of play", and the like should include thoughts on accommodations for persons with disabilities, safety tools, managing different types of players, and the like. Focus the beginning of the book on best practices for recruiting and session zero.
Things to expand
- Running the Game: It oft-noted that the "Running the Game" chapter is the most necessary, and yet is at the end of the book. They could expand this section further with more detailed procedures for how to do...whatever the new edition wants to do as a game (e.g. in b/x, it's dungeon- and wilderness-crawling, so there are procedures for that)
Yeah - get this up front. Discuss incorporating character backgrounds into the ongoing plot, maintaining momentum and interest, managing different kinds of players, common stumbling blocks, and the like. Given the probably increasing importance to WOTC of D&D Beyond (or something that replaces it), include links to (or, in print, urls for) articles that expand on these ideas.
- DM Workshop: An expanded DM workshop section, with modular rules to fit a wider array of settings, tropes, and play styles. It's probably unlikely that they would do this. At the same time, they could go through all the optional rules they scatter throughout the book, reconsider what they really need, and gather the remaining ones together in one chapter.
Yes, please! This is probably my favorite part of the current DMG, s keto this and enhance it with stuff from subsequent releases.
Needs repair and revision
- Usability: editing, organization, layout.
Always. Again, making things as clear and as easy to find as possible is especially important for new players.
Expanded downtime content!! Bring in stuff from Xanathar's and elsewhere.
Remove?:
- Miscellaneous rules: mechanics for things that don't come up that often, like ship rules, chases, diseases, etc. Would a lot be lost if rules were not included?
Some basic ship rules seems important if you want to show how wide a variety of adventures the game can handle, right alongside dungeon crawling and wilderness exploration. Chase rules seem important, too. I'd like to see some basic mass combat rules, including stats for some seige weapons, castle walls, etc. Any and all of this can be expanded on in later books, but it's like to see the core ideas in the DMG - again, to show the range of things you can do in a D&D game.
- Worldbuilding: The worldbuilding and cosmology sections are the weakest and least relevant parts of the book. I feel to do it right, worldbuilding needs much more space that would be allotted to it in a dmg.
I feel much the way I do about mass combat rules, above: a section with some basic concepts is necessary, I think, to suggest what's possible, but the full ramifications can't be covered in the DMG. And since worldbuilding is so big and so expensive, the best you can do is devote a little space to examples and core concepts like creation stories, pantheons, etc., and include a nice sampling of examples from other D&D products.
Honestly, I'd be okay with
no magic items in the DMG if it meant making room for more of the above content.
