Perhaps it would be....
However, for the coming expansion how about a world with 3 parallel story lines? For instance, applying that concept to Tyria, when you play Thunderhead keep, all those Mursatt rushing at you would actually be human players playing out their end of the story line (and doing their own missions?), likewise same for many others.
In essence, it is the true combination of PvP and PvE. Unlike what we have right now, where everything is scripted and somewhat predictable.
Plus, it has the added bonus of providing a VERY good challenge. I for one would not be tired of playing a mission for the hundredth time if I know it is other human beings at the other end...
It does take more work and some thought to implement, but I'm sure it is a good idea
