don't know which difficulties are unlocked by default but I think its probably best to start on the harder ones. this applies to all action games in my opinionBased on the IGN pre launch party show, it sounds like Ryse will at least be a very solid game... between 7.0-9.0 rating.
They seemed to have fun with it and did it say he wanted to replay the game all the way through on a higher difficulty so it's clearly not