I mean in the implementation, not the high-level design goals. What would you expect from a sandbox that is not delivered by an AP, and how would you expect a published sandbox adventure to be laid out? Are there any examples of published sandboxes? (I can't think of any, but I am also not well-versed in ye olde modules of yore).
There are lots of published sandbox modules from older editions; Sinister Secret of Saltmarsh springs to mind as a location-based sandbox. From 3e era there was Red Hand of Doom, which I would describe as an event-based sandbox.
Features of layout and implementation that I would want in a published sandbox include:
- A way of organizing the adventure/book into logical parts/chapters. For example, Red Hand of Doom divided its chapters according to stages of the orc invasion.
- A detailed home base with named NPCs and businesses.
- A list of the various quests, with notation about where/from whom PCs can get the quest, and it's rough threat level.
- A great map or three with a location key that clearly denotes the threat level of an area.
- Fascinating and dynamic encounter tables that are more than "6 kobolds armed with spears."
- Rumors which natural in-game language to cue players into just how out of their league they're getting.
- In the case of complex mysteries, intrigues, or factional allegiances, use info graphics to create clear concise flow charts.
- An alphabetical index.
The introduction: How railroady is too railroady? Is it ok to open with a framing story like "You guys are on your way to meet King Soandso who as a secret task for you. You were intrigued so here you are," or is that too contrived? I guess I'm asking, what is the best way to get the players moving in a direction, any direction? In previous sandboxes players have complained that they simply didn't know what to do next.
Having a strong framing scene, a hook to get players invested and *wanting* to explore is SO important. When I write adventures for my home games, I record an individual hook for each PC as well as a group hook. I find individualized hooks do wonders to get players thinking in character and being asking a more active role.
Your example, while a bit thin, could work as a group hook. However, the problem I have with your example is that starting with the meeting rather than "en route" would help players to focus and make it clear what's expected of them right from the get go. And just because the king gives them a secret mission in the opening scene doesn't mean that they're on a railroad.
Player vs monster level: This is much less of a problem in 5E than in 4E, but how do you plan appropriately balanced encounters in a sandbox when you don't know when or how players will tackle them? Or is this a feature of the sandbox, that players will run into things that can TPK them? Or that they'll run into encounters they can steamroll? I don't like the Oblivion/Skyrim "monsters are always your level" play that 4E pretty much required (or you were forced to run linear adventures, which is what I want to step away from), but I don't have an answer for the level disparity problem (if, indeed, it even is a problem).
It's a feature that encourages creative thinking and keeps things fresh. Especially in 5e you don't need to worry about this too much, just try not to throw an army of monsters at them.
The conclusion: When is a sandbox adventure over? My players are a big fan of free agency, but they are equally big fans of story, including the climax, the denouement, and the eventual end. But in a sandbox it seems like the ending is a lot more ambiguous, and more so the number of open threads you have going on. One thought I had was having all extant story threads funnel into one overarching epic, with all the foes they have fought along the way being pawns of one BBEG, but perhaps that's been done to death? Contrived? I don't know.
IME a sandbox has a shape similar to a spindle top. There's a short narrow top where the adventure begins (the King's secret mission), a large wide middle section (with lots of quests and places to explore), and as the players get familiar with the world, make decisions, and begin to zero in on certain bad guys, the campaign naturally narrows down to a particular story with its own climax and denouement.
Personally, I find Dungeon World's "fronts" to be a helpful aid in planning various plot threads and providing them with depth. I also connect these plot threads to each other, usually with a mind map, similar to your idea of all the villains being lieutenants of the BBEG (just a bit more evolved).
Player paralysis: With no big sign saying "go here, do this" my players are apt to scratch their heads and say, "I dunnow." And I by no means blame my players. I don't really think I'm that great of a DM, so I am either laying too subtle clues or I'm over-complicating my stories. I think it was Angry DM who once said to me on Twitter that even having a story (and I might be grossly oversimplifying or wildly misinterpreting what he meant) meant I was tacitly railroading my players. If that's the case, is player paralysis a function of their expectation that I have a trail for them to always follow? How do you instigate player action that more organically generates adventure?
A DM whose clues are too subtle and whose plot is too complex with simplistic monster-bashing players? No, no, I have no idea what you're talking about
I think Angry DM's advice to you was bunk, even if I respect Angry DM himself and enjoy his blog. Saying something is "railroading" is about as helpful to a surgeon's assistant describing an organ as "red." Yeah, ok, and...
As a specific piece of advice regarding clues, I recommend you implement the "3 clue rule" which states that when your players choose to delve into a mystery which has various clues for them to piece together, you should come up with 3 vectors they can gain each clue from. This is because players tend to miss the clue the first time, misinterpret the second time, and finally get it on the third time. As a DM things which seem obvious to you are not obvious at all to your players, so having a "3 clue rule" system ensures that they have various ways to gain information.
Also, don't be afraid to use hard scene framing. That's a conversation of its own, but IMO scene framing and sandbox gaming are not polar opposites and can actually compliment each other nicely.
Setting material: In no event has a player ever read any material I have ever written for an adventure. So is it a waste of my time? Is it still any good for internal consistency? Is internal consistency even necessary for player immersion? When you read a published campaign setting or sandbox adventure, do you as a DM actually read things like calendars, historical timelines, and exhaustive breakdowns of churches and factions? Do such things enhance sandboxes and where is the line that you've written too much?
This varies wildly from DM to DM, and from product to product. Personally, I'm a utilitarian DM; if I know or foresee us using setting information in an adventure then I'll record it, if not I usually toss it or make a brief note in my campaign journal. However, some DMs enjoy writing setting information just for love of the creative act; nothing wrong with that at all!