You can save anywhere in MO, so I'm assuming the same will be true in OW4. As for a backstory, I'm not sure it's worth it unless parts of previous games actually affect what's happening in this story. If they do have a backstory, hopefully it'll be like the one in AE and not the one in MO.
However, if it were to have subtitles, it certainly would get a higher score on deafgamers.com, plus it would be easier to transcribe!
A view all movies option should be there, too, but not so you can see movies you haven't reached in the game, and also whatever 'quarma' (if that is involved with the new game) ending you end up with, you should be able to start the game again you can end up with ALL the movies on the same save file.
And I expect to see all the OWI dressed up as hunters for the credits.
EDIT: I don't see a point in subtitling for non UK and US residents, since the games are actually translated (I believe).