I think the main story line ought to be a railroad. The coming of the Umbral Era-- it propels all our characters into the landscape for more content. Sort of like the pilot episode of a television series. Following that context, there could be any number of narrations and ho-ha's for any player-- Multiple exclusive decisions, morality, path companions and so forth in expansions or DLC/episodes-- Once the gameplay is running on it's own two feet, I think we can expect a lot from Square in that department.

I'm not implying they might use morality or character branching. Only that Square would have the perfect opportunity to prove how superficial some modern-day thinking of MMOs are. (Endless customization, voiced PCs, multiple exclusive decisions, etc. Those are great features, I agree, but don't make a MMO actually gratifying.)
Just my two cents.