Back in Ireland this past weekend I met up with my old Katharsis table and we got to talking about the coming edition - in particular what would need to be in those books to get us to lay down our cash. Running theory is that the PHB will shift huge numbers, because players love options. The Monster Manual will not because there are so many online equivalents and a significantly smaller pool of interested buyers. We were not sure about the DMG; and wondered what we would want to see ourselves - a book full of settings perhaps?
This would be a book with a chapter on each of the major D&D settings to give just enough to get started in any of them - Forgotten Realms, Greyhawk, Spelljammer, Planescape, whatever. This would make it something that both players and DM's would want and hugely increase the potential pool of buyers.
For me, a neat edition to this would be optional rules tagged into each setting - Forgotten Realms is heroic fantasy, use the standard rules. Greyhawk is supposed to be more swords and sorcery, maybe use Gritty Realism, etc. All the various optional rules would be grouped and presented with the setting to support the feel of that setting, making there be a reason to play different settings to get different experiences out of the same, known, system. I really like the way that Brancalonia is effectively a 5e mod - it gives a different feel to the setting without have to have people learn a whole new system. Things like ship combat, overland exploration and other mechanics could also be tied in as setting options.
It could also be an option to turn that ratchet another notch and have certain restrictions in settings - not everything available at all times. Taking the examples of Iron Kingdoms and Eberron which were both steampunk fantasy settings, Eberron has everything that is available anywhere in D&D within it, whereas Iron Kingdoms has a more restricted set of options that gives a different and more distinct feel. This may not be to everyones preference but having each setting chapter in this hypothetical DMG come with a list of restricted races, classes or other options would also go a good way to making settings feel distinct. Things like restricted magic-user types for Dark Sun, for example.
There is a huge range of rules from prior editions that could easily be drawn on here for flavour - expanding martial rules so types of weapons make a difference, over-clocking spells with spellburn, spell-duels, changing the basis of XP from monsters slain to gold-in-hand or carousing or training.
We talked about random generators and the like - tools that are very tied to DM style. One of us would use random generators quite a bit, another would deliberately craft encounters to advance the story line. This spread within our tiny discussion group coming from pretty much the same point of origin shows the needles eye trying to be threaded here. I think generators are helpful but would be best framed within where and why to use them - rolling random encounters alone does not a coherent campaign make.
One other point we were talking about was how this all interacts with the DM shortage - which would strongly guide the tone of the book - is it tailored towards getting someone started or providing tools for the experienced DM? Given the perception of a lack of DMs - one I find plausible based on what I am seeing at my local games society - then the current DMG is not sufficient in itself to get people to give it a go.
Can a book serve both markets? Maybe - teaching front end, settings with house rules in the back two thirds. The how-to-run section could even be helpful for experience folk by laying out the various schools of DM'ing with tips and tricks - you want to run fully theater of the mind - try these ideas. You want to run an actual play online, here is what matters. You want to get crunchy with minis and battlemaps - here are the ways to do that.
I think given the wild proliferation of house rules, there could be appetite for a book with lots of options laid out in it but the new thing would be laying out exactly which sets of house rules to use together to get what kind of atmosphere at the table - tiny spots of the design theory, the likes of which OSR blogs make great hay from.
Given my forecasting record for WotC books, all I can say is that this is almost certainly *not* what we will get but having said that, should anyone else choose to step up to that challenge I would be interested to read what they put out.