All things considered, navigating the reactors and sewers should be linear. They would be dungeons after all, most of it one time use and were already extremely linear in game. Environmental navigation is easier when you're going through a hallways, maybe.
Then again it's Nomura and I feel like in his KH experience, he would try to throw in platforming in somewhere. I'll be honest, not sure how I feel about that. Lightning Returns was a pretty open world and its platforming elements did help make the world feel more like a world, added little and easily could make the game frustrating.
The real test is to see how open Midgar is.