Here is your warning now; I am going to spoiling some major plot points and mechanics of this game including the ending so if you want to play 100% blind then avoid this thread. Whether you choose to believe this or not is completely up to you, but you have been warned.
>How much did Martin create in the story?
Martin created I would say about a good 90% of the overall plot and lore of the base world. He was some general themes at the beginning and worked off of that. The underlaying themes and central plot lines were 100% controlled by Miyazaki. Essentially Miyazaki drew up the foundation, and Martin filled in the rest and was responsible for the lore surrounding different areas of the game.
>Is this a Souls Game?
Yes and no. Technically yes, but it's being designed purposefully ambiguous enough to so you will be able to draw your own conclusions. As some of you have more than likely already guessed the "Elden Ring" is a direct reference to the Linking of the Fire. There will be a few somewhat obvious Souls references in the game, but the Souls world won't have any real bearing on what is happening in this current game.
>How open world?
It's exactly as it sounds. From the beginning of the game you will be able to go anywhere you want with no hand holding. The world is divided up into different kingdoms, each with their own culture and ruling agendas. What is interesting, is which areas you choose to go to first and decisions you make will "alter" the other areas. What this mostly means is that other kingdoms will become more difficult and questlines may change depending on what order you decide to proceed.
For example; one of the kingdoms is plagued by a mysterious disease that if you were to say go to that area first, can be taken care of fairly easily. If you were to save that area for last, the disease will have spread further, causing greater mutations and more terrifying creatures to appear. Mechanics like that.