I'd have to say that most, if not all players would be undead, maybe some un-cursed humans if the plot allows it. I say this because in Dark Souls there's not much in the way of sentient races that aren't just too drastically different from humans to work out. So probably no elves or dwarves or anything like that, but if someone can create a unique character that fits into the lore of Dark Souls, I'd be fine with that.

There won't be any set classes or anything, you can do pretty much whatever you want as long as it makes sense and isn't overpowered. Sorcery, dark magic, plain old swords and shields, anything that would go well with the plot of Dark Souls is perfectly fine with me.