Now I don't want to start argument, but Arkham Knigt is one giant sand box with zero diversity. Also Darksiders 2 also suffered from reduced diversity. And Darksiders 1 essentially splitted in levels in that.
Dude, no offense, really, don't take it the wrong way but seriously, sometimes your comments make me facepalm, cause it's like I say garlic and you understand onion, and it hurts my nerves a bit. What does Arkham Knight have to do with diversity in the context of this particular discussion? I only mentioned it to talk about the combat score system, to show you that you can have one even in an open world game.
Anyway, there are still open world games out there with good variations, Morrowind comes immediately to my mind for example. Stuff is not always black or white, man.
But, if you're not into the full fledged open world concept, how about keeping the mission structure, but making each stage bigger, with alternate routes and stuff like that. Would please those who complain about the lack of focus on exploration that DMC1 had.
Now, I don't really care that much, DMC's never been about exploration as much as combat for me, but I'm not against the idea.
Keep an open mind man, don't just ditch something immediately just because, give it some thought.
Also some levels offer storytelling through places (like Phantom chasing you in DMC1 etc.)
Now that is a more legitimate concern, but you can still do that in a more open game through scripts. They exist for this, you know.