Speculation: How to achieve SP with seamless co-op?

Obligatory I-know-we-don't-know-anything-about-Anthem-yet-and-I-should-just-be-patient and It-will-be-like-SWTOR disclaimers. With that out of the way, why not have some fun with blue-sky speculation about how this might work?

Stipulating to the possibility that it might be like SWTOR, how else might it be achieved?

Problem: if you have never done quest XYZ before and you invite friends to join you who have completed it, how do you explain away the plot discontinuity?


McGuffin resets quests for co-op repeaters

With a McGuffin. Imagine there is something about the world, like Shaper Storms, that reset quests, a little or a lot, so that even if your friends have done it before, they can't entirely spoil it and, from their point of view, it's a different quest. Maybe the Shaper Storm alters time, maybe it alters the NPCs who populate the plot of the quest, maybe it does both.

Splits up your party at key decision points, so your friends are only supporting cast/extras

This is basically the ME3 Citadel DLC "Citadel Archives" solution. There, you had your immediate squad with you, but the rest of the squad split up and was visible from time to time, or you can hear their chatter over coms.

In this case, you'd go solo into key decision points, while your friends go someplace else as backup, overwatch, or just taking a different fork in the road to meet up later. You hear from them and see them from time to time, but they can't interfere with your story decisions.

An important, and interesting, difference from the Citadel mission is that even though your friends are going someplace else, their experience of the quest is just like any other quest. It's like a quest with two branches, A and B, and while you always do A solo and only once (per play through), you might do B with your friends multiple times. True, this requires a significant suspension of disbelief on the part of any player that goes through the B branch a hundred times, and why does that dead bandit keep coming back to life, I thought I killed him when I did branch A solo?

You can't invite friends to quests they have already completed

Pretty harsh and clumsy, but can't argue it wouldn't protect story continuity. Arguably not very seamless, though.

It really is a shared world

You opt-in to an instance that you share with a specific set of friends, and that's it. Once in, state changes by any player persist for all other players in the instance. A friend unjoining that instance is the same as hitting the New Game button, and you can only join one instance per player ID.

So if your buddy goes and completes a quest you've been saving for later, you are SOL.