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Yes in a way we are taking up that tired "sandbox" vs "themepark" debate and in a way that sucks. Both of those terms are loaded with baggage that I'm hoping to avoid by talking in the higher concepts of the title. There's another reason to do this as well, I see one of the future keys to success being to merge those two elements well.
If we are being honest, sandboxes have a limited market even within the larger MMO segment. Their advantage is that when well executed they are in many ways easier to maintain and will retain higher membership (as a %). Themeparks draw a larger audience but require near constant additions to the content and even then will lose membership as players tire of the setting and the constantly moving goalposts. An over simplification certainly, but really there isn't much more to be taken away from this debate, nor does it give much insight into designing better games.
Hopefully that was enough to move us off the usual script and instead move us into a discussion about the strengths/weaknesses of both emergent and static elements and perhaps more importantly we can even touch on good ways to bring them together.
While there are easily hundreds of variations I'll start by spinning out one I recently put forward in one of Loktofeit's threads (BTW sorry for hyjacking that thread). The basic idea was to add modifiers to individual merchants and then to create a mechanism for succession. The replacement would have a new set of modifiers. The emergent portion focuses on giving players a chance to change the game world. The static element is that a merchant will quickly fill the same spot and that the dev would still be in control of those modifiers (or at the very least in control of the range those modifiers are drawn from).
I'll stop here before this gets way too long.