And this is where we disagree. I do not think that the ruleset needs to be altered. Thus, I don't think we're ever going to agree on this one.
If the ruleset doesn't change, then you have people drinking and playing chess, but no new game.
Moreover, your position here is mutually exclusive to things you have already agreed with, as demonstrated by (thus far unresponded to by you) syllogism upthread.
From a rational standpoint, given the syllogism,
A = B
B = C, therefore
A = C
one can rationally disagree that A = B, or that B = C, but if one accepts A = B and B = C, there is no rational way to then deny that A = C.
To most people, a sandbox world exists independently of the PC's and details are fixed. That spooky castle has whatever the DM has put in that spooky castle regardless of what group is playing.
I would agree to a large extent. I would not change the broad strokes. Details may change, depending upon PC backgrounds, though.
However, all sandboxes are "maleable to the group playing" insofar at least as it is the group playing, rather than the GM, that determines what the group does. And, again, the details can accommodate PC background (as exampled upthread).
Well, considering that Doug M's definition of sandbox is pretty much the standard definition of a sandbox campaign - a campaign where the world contains a number of adventures that the players can choose from at any given time - how do YOU define sandbox play?
I would disagree, because "adventures" are not necessarily discrete units in a sandbox setting in the way you (apparently) mean. Locations are discrete units, certainly, but what happens at a location is not.
Thus, if the GM takes the time to detail Giant Land, then the PCs may have multiple forays into Giant Land, and not always for the same reasons. They may cross Giant Land, they may engage in diplomatic relations with Giant Land, they may raid Giant Land, they may seek a ruin lost within Giant Land.
How the game element "Giant Land" and its sub-elements are used are limited only by the imaginations of the players.
For the GM, this has the advantage of allowing his work to do double duty. If he created the Village of Hommlet, it could be the PC's home base for the exploration of a certain moathouse, but also contains enough adventure seeds to be of use throughout the lifetime of a campaign. That is why Gary Gygax included all the affiliations of townsfolk, where they kept their monies, etc. The site is useful beyond playing as a one-shot. Indeed, it can be built upon and change over time. Even within the limitations of the original module, there is a castle in progress of being built which, in campaign play, would demonstrate that Hommlet changes.
Likewise, if run as part of a sandbox, the series A1-A4 can be broken down into elements are rebuilt. The "yellow sails" of the slavers exist prior to the PCs determining to track them down. The general locations and network of the slavers can be placed, so that the PCs can run afoul of it by intent or accident. The GM no longer expects that the PCs will follow the modular series (although it is possible that they do), and the GM will not force the PCs to lose against the Slave Lords just so that he can toss them in the slaver's dungeons. In fact, the GM may expand upon the slaver's dungeons so that they provide enterprising PCs with a "back door" into the slaver's stronghold!
Moreover, in a sandbox, the PCs may never choose to track the slavers down, or run afoul of them. The slavers might remain part of the framework. The PCs might even make use of the slavers for thier own ends. They might buy slaves, steal a slaver ship, or use the slavers as allies against a common foe. They might even overthrow the current Slave Lords and take control of the operation.
You keep claiming that the sandbox world is less malleable to the PCs than the AP, but this simply isn't true.
Even if the players get lots of choices about how the world is when created, that is a very poor exchange for losing the breadth and depth of choices a sandbox allows for thereafter.
The idea that the players choose the world when they create their characters also begs the question, "What happens when a PC dies?" Must the players also be given plot protection, further eroding the meaningfulness of their choices, or must the world be changed with each new PC? What if Cousin Susie joins the group? Do we change the world again? If not, how, oh how, is she going to achieve any depth in her play experience?
Again, how is it possible that the DM can be so creative as to match a world to 4+ players, but none of those players is so creative as to be capable of matching a character to the world? How did you cope with this when you were running (and lauding your running of) the World's Largest Dungeon?
In a nutshell, your several positions in this thread require one to accept far too many mutually exclusive statements.
RC