Yeah, my first run through is a silent assassin slaughter fest with most of them. Second round is no kill. Fulton will screw up my normal way of doing things, though.
Fulton is also an option

I mean on how to use it. You can make yourself go all role playing out and carry a guard to a remote location and then Fulton him.
I agree though that if Fulton is as broken as it was in the demo (i highly doubt it) I will avoid it as much as I can.
Exactly and this is 2 ways to play MGS. I am not only talking about branching story and quests, but rather about different approaches to the same situation. So far the most of these stealth options serve a singular purpose - sneak in, distract and knockout a guard. Distracting and knocking out is one way, while going all rambo is the other way.
But there are tons of other imaginable ways of tackling the situation, just like I've explained already - you could probably make the locals attack an outpost or obtain a disguise, so you won't have to repeat the same cycle of sneaking in and knocking out guards. Also you could misinform the opposing force, so they transfer the guards or your mission objective so you could hit them on the road, not in a heavily fortified base.
There are tons of possibilities that could dramatically alter the gameplay, but for now we can only knockout a guard with 10 different methods and sneak in further in yet another 10 different methods. All cosmetic changes, while the gameplay is still the same as is the result.
But most of the ways you have listed here, existed one way or another in multiple MGS games.
For example the disguise was used in more than one MGS games.
MGS4 also had battlefields that you had opposed factions and you could manipulate things, albeit not in a huge way.
Also, in general MGS doesnt have just 2 options knockout and distract. You kind of generalize it , and I get why you do it, but certainly there is more to this than that.
There are multiple ways to distract an enemy and multiple different outcomes depending the distract method you use. Knocking out guards also has similarly different outcomes, depending on the method used. You can knockout a guard simply to distract another guard.
Also, again, going open world with real time weather and a vast area at your disposal changes completely the way you can approach a mission. You can even bypass entire sections. Some people may go head on battle against the main area of the objectives, others could scout the area and find secret passage through a nearby mountain or village. The day/night cycle affects the shifts of the guards, so you also have extra options there that affect your approach, you try and go during the night with darkness covering you, or broad daylight but fewer guards? And what happens if a sandstorm comes out of nowhere and messes with your plans? And lets not forget that Kojima always likes to provide gameplay senarios that change the conventional approach we expect and keeps the game fresh.
All these things really make MGSV an engaging and always changing environment, and I believe this is enough to make everything stay fresh and give players the options to approach the situation as they please.