With additional Story Content portraying a linear passage of time by incorporating events from older Quest zones, the damage to the game's timeline is only being worsened, and making it increasingly more confusing and less immersive to newer players who are trying to grasp the game and it's story lines.
Because of this, I'd suggest instituting a Content Lock system.
Please, lay down your pitch forks and delay your furious keyboard smashing replies for a moment. Because of One Tamriel's "Play whatever you want, whenever you want" ability, it is directly impacting the game's linear story telling by letting you experience not only future events first, but also both future and past events at the same time,
for example, this thread. When one moment you're being told by the Companions that Tharn must be recovered from Mannimarco's Castle, but then you go to another location and see the man is perfectly fine and asking you to help him find the Demon Weapon, that's a major conflict.
In the interest of preserving the game's story integrity, the Content Lock would ensure players experience all game content, from the Base Game up to it's latest Chapter and Adventure zones in Chronological Order. The Content Lock would place Prerequisites on prior Story Content, only unlocking once your character has completed the previous Main Questline. In practice, the game won't allow you to enter The Clockwork City until you've saved Vvardenfell, and it won't let you enter Vvardenfell until you've stopped King Kurog in Wrothgar.
As an added bonus, the Content Lock system will give players a sense of encouragement to Quest, as well as accomplishment when completing a Questline which then opens up a previously closed off land to them. Yes, restricting content sounds unfair and unenjoyable, most new players that are enticed into the game are only here because of the hot new content such as Summerset and Elsweyr, but with the way ZOS is writing the stories for these Zones, no favors are being done to them when it comes to grasping it. Playing the new content first will only lead you into Paradoxes, and then we get more and more threads about Dragonbreak pish posh.
In order to create a better storytelling experience, we must sacrifice a little gameplay freedom.
You may now smash your keyboards at me.
Another less restrictive idea.
If not an addon to the Zone Guide then how about just a Main Quest lock? The zones themselves can be available to explore and quests associated with delves, and the world are open but the main questlines would have prerequisites