What I am attempting to point out is that with no prep and no mechanism to rely on for determining how your improv should go, that ultimately you have nothing to base your in play GM decisions on - they are completely fiat. I question whether a gamestyle that is based on complete GM fiat can appropriately be called Story Now.
I'm also thinking that what D&D players call sandbox is often very similar to Story now (just with some added world background so that in game decisions aren't completely fiat.
But that doesn't change the fact that it's complete DM fiat to improvise a combat that looks like this as opposed to some other way.
If skill challenges are an answer to my criticism your not doing a good job of explaining how.
This one in particular sounds very sandboxy to me.
If your saying 4e allowed story now and this is one of the reasons, i'm starting to wonder if maybe all sandbox play in D&D qualifies as Story Now using your definitions.
Here is what your thoughts above are missing:
Let us say that at the apex of the "fiat decision tree" (lets call it), I have a creative menu of 1000 possible items. Complete GM fiat (no constraints, no boundaries, no player say and no system say...ALL GM SAY).
1) We have a constraining premise of play.
That creative menu of 1000 possible items is now winnowed to 200 items.
2) We have table-facing and transparent machinery which further winnows a GM's decision-tree (I can't choose framing and action resolution results and consequences that are violations of this stuff).
That creative menu of 200 possible items is now winnowed to 50 items.
3) I have player-authored Quests, Theme, Paragon Path, Epic Destinies and prior fiction which all must be honored as the volitional force of play.
That creative menu of 50 possible items is now winnowed down to 25 items.
4) I have a game with clear principles (cut to the action, fail forward, follow the fiction and change the situation based on goal of conflict) and tools/techniques for those principles to integrate with all of the stuff above.
That creative menu of 25 possible items is now winnowed down to 5 items.
That is BS math, but that is the reality of it. In any given instance of play, your GMing decision-tree (in situation framing and in consequence adjudication is going to be dramatically winnowed down from MAXIMUM FIAT because of all of the constraining factors of the system which winnow your cognitive workspace and focus your creativity to a handful (ish) of "play-honoring framing and outcomes."
You then just follow that stuff and keep play orbiting around Quests/Themes/Paths/Destinies.
Its simple.
And its HARD to do the opposite (impose metaplot despite all of the decision-tree constraining/winnowing aspects of system).
So...again...why would you?
There are games that make it very easy to impose metaplot and consistently situate play in that MAXIMUM GM FIAT space. 4e isn't one of them. So play one of those games (this is where edition warriors say YEAH I KNOW WE DID)!