Ok, so let's recapitulate:
Tranquillity, Fridén and Fredrik are the Skydancers (Brändström being the youngest), making music for the universe (i'm thinking something Silmarillion-like, where the gods create the universe through their music or song). They all have different powers, all equally great (which powers?).
The Mind's I seems to be some superior power that created them (we need to explain this better).
The Void is the essence of the universe and of the very concept of Tranquillity (and later DT, but not In Flames) itself.
Fridén betrayed Tranquillity and took with him a lot of followers, thus dividing the universe's population into Light Tranquillians and Dark Tranquillians, from where In Flames and DT were formed, respectively. Then Fredrik did the same, resulting in what? Also, we need a battle between Light and Dark Tranquillians (and potentially another battle between Dark Tranquillians and Fredrik's faction).
Another important thing to include, i think, is the downfall of In Flames & co. (in the form of the supposed decay of the quality in their music (i still haven't listened to new In Flames)), showing the superiority of DT over all other things.
Another thought: There could be "three kings", which would be different attempts by The Mind's I or whatever to create the ultimate gods/whatever: DT, IF and At The Gates. There could be a big war resulting in the death of ATG and the weakening of IF or something.
Am i missing something?