Due to the lack of definition, I don't even understand what the OP wants.
I fail to see what practical difference OP has in mind. Seems to me that if he wants to focus on "secondary character" questline, and force the party to fulfill this one first, he can. He can even stop the game once it's done. What are the mechanics the OP wants?
I think I described it pretty well, but I'll try again.
I want to create four characters, each with the ability to make their own decisions, and pursue individual and even incompatible goals. I
do not want any of these four to be "the" player character; none of them should be the Chosen One. It should be possible for any of the four to succeed or fail at their personal objectives. It should be possible for them to take different sides in a conflict and work against each other. It should be possible for any of the four to die (perhaps even by killing each other), and the game will continue as long as someone is left.
From what Swen described, none of this is in the (single player) game. We can only make one player character, and this character
will be a "Chosen One"; the game will center around his personal story, and any companions he acquires will be subservient to that story (if they
do have goals incompatible to the hero's, they are doomed to fail). The party will all agree to pick one side of any conflict (probably whatever Our Hero decides). PCs #2, 3, and 4 will
not be able to pursue their "origin" quests. If PC #1 dies, the game is over, making him "essential" while the rest are just sidekicks.
