So some of the ppl here understands just the half of the fact, yes, there is 2 main regions, but you should really understand that each region bigger than a whole game, and you call it not an open world game? What is it then?
When each region would stand on it own in a complete different game. If you would like to call a game, which was called an open world game, that would be the DA:I, for me an open world game have dinamyc weather and day night and none of thees are in that game.
I would like to write some infos about the MGS V open world. I know that it will be off topic, but someone mentioned it before. Kojima has only showed us a little part of his open world maps. Some of the Kojima station episode mentions that you will able to do multiple missions in one deployment, you do not need to go back Mother Base, you can explore and do some main or side ops as you will. Of course you will have limitations during missions, but as soon as you finish it, you will be free to explore anywhere. And here comes my guessing about this, Lets just say that you go for an intel in one base, and you raised alarm, it is possible that as you approach a different base in that place they will be on higher alert.