There's certainly ways to get around the whole optional content issue.

-Make the Shb role quests open to any role, the reasons were already flimsy and it's not worth locking them away from players who don't want to branch out.

-Create an extra scene or two with Unkullhai heading to the first to force him up to plot relevance of players haven't unlocked relevant stuff.

-Time shenanigans. If the 1st was moving at a faster rate to the source during SB patches then reason goes time has likely started moving quicker again during the endwalker timeframe. Perhaps they could appear because the few years needed to investigate voidgates has been completed and any interactions with them from non-completed players could go along the line of "oh, this was before we met" like Graha in Shb.

-Let it rock, just don't acknowledge it estinien style, after all half of the trial/raid quests use scions from earlier points in time. Maintaining continuity of the game world isn't nearly as big a priority for devs as some think.

Also just as a fun coincidence I noticed.
-Thancred/urianger are heading to this "new location" as per 6.4. They are the ones involved in the Eden questline.
-Mercydia is down south, So is the section of the empty explored in the Eden quests.

Anyway I'm sure alot of us are going wild with predictions for 7.0 and the void, for myself though it's not just a matter of matching semi-relevant characters to every plot thread. It's more that, from a marketing perspective I can't picture anything else. There are multiple links and intrigues relating to the 1st whilst every other lore well has been drained dry, Endwalker basically capped off all the locations fans had been curious about in one fell swoop without leaving any room to expand. The patches could have set seeds for something new but with only the 6.5/55 patches left there's no time to create a new source of intrigue, leaving it up to what we have left (I.E. The 1st, Unkullhai, cyella).

Because let's be honest, if the big selling point of 7.0 is "Getting to visit a continent nobody cares about with Krile and Zero", then the MMO will die.