Your theory of Vessel seems pretty odd to me. I mean, If it's the Ascien that do it for getting us out of the way, why didn't they do that from the beginning ?
In my opinion, the main story will take somewhat a Space-time way, by going through a new world, or going back in the time.
But, with the new map that have been shown we can't be sure. Rak'tika Great wood seems to have a Viera village in it, So Ivalice/Dalmasca will be our next destination ?
To stay in the theme of Map, we got 2 new destination on each Expansion
HW was Dravania and Abalathia
SB was Othard and Gyr Abania
So Shadowbringer will give us Ivalice/Dalmasca and ???
Don't forget that travelling in the time can destroy the entire universe. Imagine if you kill, for exemple, Hien before the war of Doma. People of Doma won't oppose Garlemald and immediatly attack you even if you try to make things for them. History became impossible to complete, thus creating a Paradox where you never reached the point of history where you need to go back through time.
So going through a new world seems to be a good option. But beside the reincarnation, I still can't explain why Minfillia is here.
So another theory appear, We are going in a world where the time is not the same as us, and where Darkness almost never appear. And look in the Eyes of Minfilia, for me it's not Minfilia, but Hydaelyn it self ! Minfilia don't have eyes like that in normal, Only the version posseded by Hydaelyn possess it.
Too many question are still there, and without the little puzzle piece missing we can't make everything look smooth. Maybe the 4.55 will open a new possibilities.