Remathilis
Legend
I think there is a very open market for a campaign builders guidebook that expands on the DMG and gives tools and resources for making all types of campaigns. Encounter tables, dungeon traps, downtime activities, monster generation, puzzles and riddles, etc. You can have it discuss sandbox and linear play, world building, and such. It would even be a pretty decent 3pp product if WotC doesn't have interest. (Although if WotC wants to jazz it up and sell it, I'm sure a few new origins, monsters and traps would sweeten the pot).I haven't read through all the DMG, but given the tone of 5.5 and the obvious focus on purchasable adventures from WotC (the main point of backwards compatibility I believe), I expect you're right about this. I find it very disappointing, since sandbox emergent story play is personally my preferred style and I hate to think of new DMs not being exposed to it in the latest iteration of the game from which I learned about it. It is, I think, more efficient financially to focus on the big adventure path products, which are also easier to run (I freely admit doing sandbox "right" IME takes real effort). So I understand WotC's reasoning here. But to not mention it at all in the DMG is disheartening.
As far as what I would add: every game in Kevin Crawford's ...Without Number series includes an excellent essay explaining and in support of sandbox play, so I would start with something like that. I would additionally release a product (sooner rather than later) that discusses and focuses around the worldbuilding largely downplayed in the 5.5 DMG, including how that supports sandbox play and also including what mechanical changes to the game might facilitate that playstyle. The missing monster creation rules would be a good fit for this proposed product.
Please note that I am very much trying to be fair to WotC and to fans of its current offering here. These suggestions are just what I consider to be improvements to their game that would expose new DMs to what I believe to be a very fun playstyle, one worth preserving for new entrants into our amazing hobby.