I'm not a big fan of metaplot. The main problem is that it promotes a "magazine-like" approach to the setting, where later material makes earlier material obsolete. Maybe there's an adventure that takes place in war-torn Tethyr, but now the True King is back on the throne and all is good and peaceful again, so the adventure no longer works without significant re-working. Or maybe I was doing something with Khelben's relationship to the Harpers, but oops, all of a sudden Khelben has left the Harpers over a disagreement and brought a bunch of folks with him to start his own spy network.
I think metaplot mostly appeals to those who buy game stuff for reading, but don't have an actual game so they get vicarious pleasure from keeping up with the metaplot.
The approach used for Eberron is much superior, IMO. All the sourcebooks present the world as per 998 YK. At this point, the setting is full of possibilities. The Mourning happened 4 years ago, and whatever caused it might happen again. The Lord of Blades is amassing warforged forces in the Mournlands, where fleshy armies fear to tread, but no-one knows if, where, and when he'll go to war. Aundair might be looking to retake the Eldeen Reaches, who took the opportunity to secede during the Last War when Aundair had better things to do with their armies. The same goes for Breland vs Droaam or Karrnath vs the Talenta Plains. The various Lords of Dust are plotting to release their fiendish Overlords, and perhaps one of them have plans that are coming to fruition. The Inspired of Riedra are gaining political influence by helping the remaining Four Nations rebuild from the Last War, but they may be working on ways to bring Khorvaire under their thumb like they did Sarlona. Or maybe the world remains in a tense post-war state while the PCs are having fun plundering ruins in Xendrik. All of these, and more, are possibilities for adventure offered by the setting in 998 YK.
But if the setting were to move forward, the developers would have to decide which thing was happening, and how that would affect the setting. And then when we'd get the sourcebook on the Eldeen Reaches, it's full of stuff about how the war with Aundair is affecting the place, and how the place has been corrupted by the Children of Winter offering their forbidden magics to use against Aundair's invading armies, and so on.
Now, what I would prefer is that the hypothetical sourcebook on the Eldeen Reaches explores the place as described in the core setting book, provides more detail about the various druidic sects, what House Vadalis is up to with their house HQ and magical breeding techniques, more about the capital town of Greenheart, some stuff about Oalian's history, etcetera. It would also be pretty cool to have that book explore a hypothetical war with Aundair, giving DMs some ideas on how to handle that. But it should not be a default part of the book, just one of many optional ways the story can go.