[@ScreenAcne] I'm a bit busy right now but I'm a big fan of the Dark Souls games. I'm also one of the (relatively few, I think) people who actually went out of their way to research the lore behind the games, as it's not very clear in-game alone. I've never played Bloodbourne, but I have a very good knowledge of Dark Souls 1 lore. Basically, too busy to take part fully, but as a fan I can co-GM if you want. 

As for ideas, the Demon War and the other parts of the initial fall of Light (Dark Souls 1 takes place long after everything's gone to hell) could be a good idea. The travels of Bishop Havel, The Rock could also be interesting. Ariamis has always been an interesting character never actually shown but mentioned in the game - she created the Painted World of Ariamis - so something involving her could also work.