Demon's Souls definitely needs representation. While Ostrava would be a good character, I think the most iconic Demon's Souls character is definitely the Maiden in Black. They could create an entirely unique and original moveset for her and give Demon's Souls a much-deserved representative.
Fat Princess is representative of the game itself. Where the game was far more team based than individual based like the others. So how they do that is that they have you directly control a neutral character who commands the team to fight for you.
In Demon's Souls, you're in solitude facing the unknown. Well, most of the time. And we can make a pretty interesting character out of this.
And I thought EVERYONE in Demon's Souls was a demon or something like that. From Software is obsessed with those kinds of plotpoints.