how do you run a game that doesn't have a line?
Ok, so it is rather straight forward:
For the first session you do need to lay down the basic premise of the game world, and some initial hook to get the PC's rocking and rolling.
I will use a Classic example: B1 keep on the Borderlands is often used as a campaign starting point. (It can be set down in lots of D&Dish settings).
A simple premise being that the PC's have decided to seek their fortunes on the borderlands, the Castellan of the keep is rewarding sellswords that help against the goblin and orc incursions. Thus freeing his men to patrol the trade road.
You have several factions: Bandits, lizardmen, the necromancer and monster factions on the caves of chaos. The you have the Castallan and several NPC's in the Keep. All with their own goals and agendas. All in a constant state of flux once the PC's start interacting with the setting, Npc's and Factions.
So the PC's may start out on a simple orc hunt, but once they start meeting and interacting with the other NPC's and factions in the area the game can go in many different directions. Maybe they meet the bandits and successfully parley with them. Join them or take them over? Do a deal with the Castellan with their now bigger forces? Maybe they don't like the risk/reward of orc hunting. So they decide to rob the Keep? Maybe they encounter the necromancer and find out that he is conspiring with some high ranking NPC's in the keep to destroy it - what do they do now?
And in the middle of all this what about those pesky lizardmen?
The options and paths are near limitless. And if they decide to follow up on that rumor that the village of Homlet is having troubles, they may decide to go take a look. Or they might not. It doesn't matter. As the GM I am managing the chaos created by the PC's. Having the factions and NPC's reacting to what is going on during play.
There is no need for an overarching storyline because the PC's are in a setting of adventure opportunities, and are free to go and do what they want to improve their lot in the world. Causing all kinds of varied consequences, mayhem, and in-game fallout in the process.
It is very player driven campaign play.
except in YOUR world the PCs are interchangeable and will always follow what the last PCs did... and as such no one of them matter.
This is incorrect. They are fully able to go in a completely different direction.
"Screw this, Let's go be Pirates." Is always an option.
And unless the payer is playing every PC as a clone of their own personality; each character is different, influencing the party in different ways.
FYI once you admit to one true wayism you admit to not understanding D&D
Seeing as how I had to explain how running a game without a storyline works, I think I get it pretty ok...
Nobody has suggested this... this is YOUR idea
Not my idea -
"...the GM does not have to resort to system manipulations in order to serve the outcome of any preconceived 'story'." - is a reasonable extrapolation based on other posts.
Rather conveniently like these:
There are plenty of systems and methods in which PC death is only an outcome if the player chooses it to be one.
My apparently super-fragile campaign lasted two years and three months without killing PCs or bowing to the will of the dice.
Which leads me to:
you simply play in systems (or modify one) where the lethality is lower.
^This^
A lot of groups would be better off going to a different system that does the style of play that they want from the jump. Rather than having to fight against the system to get the in-game results that they want.