I see many point about the Source, but this isn't the expansion for me.
Maybe this will represent like 10% of the story via visions or roleplay.

So, I think, and this is what I like it, is we don't know what will happen on the First.
What's really the story of the First, and of the Flood of Light ? Is there a link (pact) between Eulmore and the Sin Eaters ?
Will we met a hierarchy in the Sin Eater ? Is G'raha Tia still good ? Will there be other (new) ascians playing a role too with Solus ? etc.

They have materials to deliver us good plots and stories.