Sound is probably going to be explored more heavily in future stealth games, maybe in a way like this, where your actions have different levels of noise. It'll probably be one more component of advanced AI and the engine behind the game. Right now (looking at MGS V:TPP) it's basically speed that determines how "loud" you are. Without the Sneaking Suit, your footsteps can make a fair bit of noise when walking slowly, but the enemy is ignorant more or less. Walk faster or run and they'll spot you no matter what. MGS V does add objects that can be tossed or broken as you walk (empty propane tanks, plastic containers, etc) which raises your noise level, but not by much.
I don't think the next big stealth game will focus entirely on sound for the AI, but it should be a bigger component than it is now. I think the sight system should be more challenging for the AI. In MGS V, they can barely see 50m in any direction unless you start shooting your gun. Even then, only snipers can really spot you if you're not running. I'm pretty sure I'd notice Snake walking around Afghan territory at 200m, both in real-life or in game (at least the way I play). If they acted like real soldiers on the field, the game would definitely be a lot harder; developers have to balance challenge and realism after all.
I don't think the next big stealth game will focus entirely on sound for the AI, but it should be a bigger component than it is now. I think the sight system should be more challenging for the AI. In MGS V, they can barely see 50m in any direction unless you start shooting your gun. Even then, only snipers can really spot you if you're not running. I'm pretty sure I'd notice Snake walking around Afghan territory at 200m, both in real-life or in game (at least the way I play). If they acted like real soldiers on the field, the game would definitely be a lot harder; developers have to balance challenge and realism after all.