There is different choices here:
- special snowflake
- Cassandra knows that in The Game it's way better to have random person in charge than yourself.
- the previews are actually lying, the Inquisition is actually a triumvirate of Cassandra, Varric(or someone else) and the player (or a dual leadership of Cassandra/Player).
- Someone else (like the big bad) actually force the player on top in the hope that the Inquisition fail.
- I'm sure they are others.
I'm going with "special snowflake". My bet has been that the sole survivor will get some special powers related to the Veil. According to the GameStar preview, there's at least some power available for the Inquisitor that consists in ripping the veil and summoning demons. Ok, it could be just another special mage power, nothing is confirmed yet, but it makes my suspicions stronger.
However, that would explain why the Inquisitor and not Cassandra leads the Inquisition. My question is, why does Cassandra help at all if she's not in charge?
Wild theory: the Divine dies at the beginning of the game. The rebirth of the Inquisition was her last card in case everything went to hell. Now the Chantry is in shambles, and while the Grand Clerics are fighting against each other, Cassandra feels that her duty is to make the Divine's last wish into a reality. Even if that means working with obnoxious, selfish renegades or blood mage pagans (up to a cinematic crisis point).