Hopes for the added stories: why did the other four fighters enter?Minor spoilers
The biggest hope is fuller stories for Bertrand, Talimun, Orchel, and Anatolia, including why they entered the Heroic Games.
Original video: Is a true Fire Emblem Octopath Traveler about to arrive? Fire Emblem: Fortune's Weave; Bilibili creator Zhugusheng Official. Watch the original
The Expansion Pass releases in three waves: the first gives each of the four protagonists an additional weapon, with outfits, gods who can join battles, and stories for four other important characters coming later. What I most want is a clearer explanation of why those four entered the games.
Why the four fighters entered
What makes the announcement most exciting for me is more story for 
Bertrand, 
Talimun, 
Orchel, and 
Anatolia. While playing the base game, I kept wishing these four could get fuller stories too.
Why did they enter the Heroic Games, and what wishes do they carry? I want to know how these important characters reached this point. Hearing that their stories are coming gives me that excitement of finally bringing together all eight journeys.
How the added chapters might be organized
I also wonder how the additional chapters will work. Will they give the other four full daily lives, world-map exploration, and unique activities like Part 1? Or will they be smaller war story chapters, moving from battle to story scene and then the next battle?
Expanding both their experiences and gameplay would come closest to the fuller picture I have always wanted.