Since it's relatively open world I've been wondering if we can pretty much get them whenever we want, in whatever order providing we don't go beyond a certain main quest or something that could lock them out (kinda like how you could miss Isabela if you didn't get her before a certain point).
That's possible, but I think it might be like this: several, but not all areas opened at the start of the game, and more opened arte the Inquisition gets stronger and we can go on the main quest (as it was stated in GI, before advancing the main plot we have to strenghten the Inquisition).
Which kind of makes sense, since at first our Inquisition might not be strong enough to reach all the areas available.