Suggestion: Leveraging SAB
Developing a “dungeon generator” would take several years of development since it is not built to be used by players. We will never see something like this on GW2, ever.
Your idea would not really work in the game.
The OP’s idea is really less about SAB and more about being able to participate in short-term content (as opposed to repeating content, e.g. SAB, Wintersday, etc.).
If ANet wants to do this, all they need to do is add an NPC that people could speak to. “Do you want to relive the events of the Molten Alliance’s attempt to take Ascalon by storm?”
As noted above, I agree that the so-called creative mode is a feature that fits an entirely different type of MMO. GW2 is “amusement park” style: the rides are already setup and we choose which ones we want to try out. The “sandbox” style game offers the opportunity for players to manipulate the environment in a way that others can also experience it.