I think you've all circled a point that could be condensed a touch: a focussed sprawl.
Like a too-night narrative is CHALLENGING, and a big sprawling narrative like A Song of Ice and Fire is manageable, but keeping a tight narrative across many authors, whilst also keeping it creative, dramatic, and moving with the times?
In that respect, you're wanting to hit "key points" and have sockets that thematically link one novel to the next, building stakes and arcs md themes, but also so that each novel can sortof stand alone, without closing off too many paths for other people.
E.g. like Chris Wraight getting lovely novels out of "why do the White Scars seem to do nothing until they're on Terra?", or "what does everyone that's not a Space Wolf or Thousand Son think happened at Prospero? What is their response?", thereby giving some good exploration to the Great Crusade, to Ullanor, to the Chondax campaign?, to razing of Prospero and to the interior structure of what happens - and why! - during the Heresy, and how all the pieces move to where they are, nearing Terra I'm a tense, all-stakes race, not a "these are the details, playing out inexorably because of predestination".
Actually, my boiled down approach could be scathingly summed up as "write Heresy like Wraight, and unlike McNeill" - which is unnecessarily brutal, but does capture the point.
McNeill's tremendously readable, and turns in great characters and stories.
But his stories largely close doors on plot points, rather than open them. Isstvan V being basically done as a quickly skipped through setpiece in "Fulgrim" encapsulates this, but so does the work of "Vengeful Spirit" (why show any development or progress of Horus when instead you can have a magic door?), "False Gods" ("Erebus is lying to you" - *finds out Erebus was lying* "Oh well, at least he didn't betray the word of my Father to warn me. I'll side with Erebus, Magnus, and betray my Father. I guess I'm evil now? Hooray, Chaos!") and "Crimson King" (what if we re-tell the story that is already written in the Ahriman trilogy with less coherency, long before its relevant, and with TONS more Sisypheum Crew & Lucius the Eternal for no reason at all?)
Hell, "Fulgrim" managed to close the door on Ferrus Manus' entire story, without actually telling any of it.
On the upside, for all that I moan about False Gods, for example, he was also the only writer to really touch on Horus' grievance with Malcador, the Council of Terra and the too-early efforts at transitioning to "this is an empire that exists forever", when there is still huge threats out there.
It took until "Slaves to Darkness" for French to pick up that thread again by perhaps implying that a lot of Horus' effectivess was enabled and realised more by Maloghurst tempering and translating for him. Horus is a brilliant mind, but he wasn't a spider orchestrating all the boring espionage threads - so someone like Malcador would always have been point some contention. Being able to fob off a lot of Malcador's nonsense onto Maloghurst would have freed Horus to get on with stuff... But none of that was explored in much detail or characterisation, and Regulus & Erebus inherited the "meddlesome puppet master ambassador" role, until they were themselves sidelined, and only at the very penultimate book in the series did Maloghurst come back with vengeance.
In that respect, we'd need more of the "oh, I see how Maloghurst fits into things, that's a fun thing to explore", and some discipline around not being too tightly focussed on telling the entire story all in one go?
Like a slower paced, or at least slightly more focused level to the story works - it allows for more sprawling.
Look at "Know No Fear" - it's a story that essentially launched a quarter of the Heresy by breathing life into Ultramar. (Well, "Betrayer" maybe precedes that.)
But the downside is that the "focus" on those stories lost momentum, between KNF and Unremembered Empire, it's an awkward wealth of Imperium Secundus stories and Calth-adjacent plots. (See also Vulkan Lives, Deathfire, Old Earth - Guy Haley and Nick Kyme managed about as much in a few short stories about the Salamanders as were "relevantly" achieved in three novels and a few novellas. In that respect, it's almost a "discipline" thing - refraining from telling the stories that *could* be told, and instead focusing on what *should* as aspects of the series?)
But then looking at a tighter thread, the quick run of Pharos/Angels of Caliban/Ruinstorm actually ties things really well.
(Actually, "Ruinstorm" itself would almost work best as a direct follow-on to KNF - just a skip of "some story happened here that's REALLY INTERESTING" but we'll tell you about that later. And that's despite me quite enjoying UE, Aoc and Pharos! With a bit more )
But you get the idea - you don't want things to be TOO tight and rigid (otherwise you get the Calth duology and essentially no Heresy for a year), but equally just commissioning novels for box ticking purposes doesn't exactly work either.
"Buried Dagger" being one of the stories that - for me - felt SO divorced from the rest of he Heresy. In terms of opening and closing doors for other stories, it's lucky that it was the last in the series before the Siege. It's ambivalence towards what else is going on in the series seemed quite... Acute.
But to circle back: it's a focussed sprawl. You want readers and authors to be engaged in the stories, and to be excited about seeing where characters and stories and world-building is going... But you don't want it to be so aimless that people get disaffected with it, or disengaged from the process.
(It's easy for me to point at McNeill's entries as somehow "out of step" - but I've absolutely, categorically no understanding of the mechanisms behind all this. For all the difference it makes, I could be finding patterns in clouds.
Writing and collaborating on novels might just not work that way, and the disciplined sprawling focus I'm suggestion might not be a thing that's practically "pursuable" by design.
But it interests me nevertheless! (And desperately makes me want to reread the whole lot!)
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