Relational Character Design in Larp

Ever tried to sit down with a group of your fellow combat larpers and create a group with a cohesive story? Everyone has their own ideas, and they are often at odds. One player wants to be a pirate, one a Stark clone, and of course there is the steam punk. Somewhere along the line you pick up 'Dark Elf Loner' #362 How do you get everyone to agree on one theme which coherently tells a story? I've enforced it top down before, but that rubs some players the wrong way.