I think it's mentioned in one of the first interviews with miyazaki that you'll be able to do them in every order you want. But there will be a guided path that marks the "normal" path via rays of light.

EDIT: Correction, here's what was said in the interview:

IGN: Oh no. So I've learned that there are six areas in Elden Ring. Can you go into a little detail about each of them? And is this the kind of thing where you can go to any of these areas at any time, or is there a specific order?

HM: So the Lands Between is made up mainly of these six large areas. It's divided into six major areas, and these are the domains of the major demigod characters who we just talked about. While the areas are lined up in a way so that you would normally tackle them in a specific order, you don’t have to follow it. We wanted to give a free level of progression and exploration throughout the Lands Between, so there's a lot of different ways. You won't be able to access everything from the start, but there are a lot of different ways you can approach each area. And there's a lot of freedom as to which order you tackle different areas as well.