Some more stuff I forgot to add last time.
Gameplay: Some mechanics that could've been in the game and improved from Peace Walker, and in general, such as the ability to lead a team of soldiers on stealth missions in the field, arena-like heavy combat, sneaking through a whole giant structure like in the first two Solid games, and even more CQC moves. More gadgets and tools that are automatically taken into the field, or not, such as a prototype grapple, charger, flashlight on the chest, upgradebale backpack, gas mask, first aid kits, and a Swiss Army Knife. Destructible environments, and a mechanic that shows the world around you degrading in relaton to destruction and resource farming you cause (for example, using missile launchers instead of tranq guns menas a forest area will be bombed out for a while). Bringing back lockers to hide bodies in. Bringing a sense of scale on top of the already large world, like say a Call of Duty game , or the Uncharted series, where it looks really big. Hiding in rafters, blind fire ability, other stuff. Lot more enemy types (based on things like Digimon, X-Mne legends, Jak and Ratchet series), but also more individual and unique personalities for them. "Demon Mode", a mode where a hallucinating Venom Snake is able to inflict much more damage on the environment and people around him, while taking on a bloodstained apearance similar to Demon Snake, and the environment looks like fire and hell on earth. Use of environmental hazards to take down enemies, such as windows about to burst in a storm, and bashing an enemy into a generator to kill them, and also ledge takedowns. Ability to pick up bricks and bottles for distraction or combat, and basball bats, pipes, crowbars, old boards, machetes, hatchets and swords for combat uses. More climbale walls and cliffs, alogn with building climbing and wire hanging. Play as Ocelot, and thinking of Mortal Kombat X where Troy Baker played Erron Black, a whole roster of playable characters available once you complete certain missions, each with a wholly different playign style to Snake (not like Mortal Kombat where everyone has the same health and button control scheme). Exponentially more options of how to complete a mission to make it "total strategic freedom". Bringing back health and stamina bars. And the ability to swim. More airstrike variety too. Ability to lure certain factions into the same area and have them fight it out. Mother Base's customization increased exponentially, and the ability to have to defend it rather than the FOBS in person. Get rid of the Extreme versions of missions, but since doen correctly they're kind of cool, make them into challenge mode kind of thing. Some original Subsistence missions, or something like surviving on limited supplies like in The Last of Us. Creatures like kaiju. "Skull mode" where Venom hallucinates himself as Skull Face, as a method of covering his own guilt at his actions.
Missions: Some possibles: Raiding a UNITA or MPLA camp to free child soldiers; assassinaitng a country's minister of something; huntnig down XOF and Cipher officials and hight value targets (for example an enforcer for Cipher who ensures. through blackmail or force, that a certain nation will secretly stay in Cipher's good graces); escorting a group of injured POWs deep in an enemy forest to an LZ due to losing your stash of Fulton devices; destroying jammers in a city area so an airstrike can be called in; clearing out a port in a war zone city for a client; escaping from a city in the midst of a serious riot; sabotaging guard towers and ICBM launchers ar a missile site; destroying a series of tunnel entrances; infiltrating a remote base where Cipher is developing new weapons of mass destruction and superguns; crossing through a trench line to sabotage soemthing; explore the Cyprus hospital once again to find some intel XOF did not know about before they realize it; infiltrate a nuclear reatcor complex; Venom heading out into the Afghan desert to find any sign of Quiet's remains, all while trying to hold in his pain at losing her; boss battle with a parasite-infected lizard that escaped from one of Skull Face's facilities;
Story: For one, lot of themes, such as gangsterism of big nations in history; the roles and purposes of nations; causes of wars such as greed and arrogance and revenge; the loss of original reasons for wars over time until they just exist; economic troubles; authoritarianism; drugs usage; the seeds of chanegs that contribute to the end of the Cold War; America's superpower status and moral self-righeousness (like in the original story plan); fatalism and nihilism at a world secretly controlled by forces and thugs beyond anyone's control; desperation of humanity to survive; the cusp of a much more techno world, as seen in the rest of the games set after this. Go more into the Metal Gear universe, its history, topics such as Skull Face's corruption and how Diamond Dogs helped shape world history. A lot more going into the themes of PAIN and RACE AND REVENGE, as well as how the first Big Boss becomes a giant prick, woven into the story structure. A lot of subplots to add to the main story, and I'll go hog wild here if I can with some ideas: 1)The return of the biological son of the original Bog Boss and a local prostitute from a brothel trip made by Jack and some of his friends before he shipped off to Korea, now intent on finding and meeting his father. 2) Diamond Dogs finds itself teaming up with a splinter group form Cipher, "the Purifiers", who believe in the original goal of Cipher but not the methods and what ever it's become nowm and a fragile alliance begins (and does eventually end). 3) Diamond Dogs goes up against another Cipher-controlled organization, this one much more like a covert mafia or corrupt multinational corporation, using blackmail, bribes, graft, threats, even hiring private military contractors in war zones who officially work for actors in the regions, but ultimately answer to the organization's economic and power interests (kind of like the RDA and mercs in Avatar). 4) A new mutation of the parasite emerges after the Shining Lights incident, which produces even more zombie-like effects, and the Dogs get into conflict with a rogue PF army that is looking to take it and use it themselves. 5) Flashbacks to such things as Kaz's flashbacks set before the game, in places like Grenada and Northern Ireland. 6) More confrontations with Skull Face, including sneaking into on of his bases, in the process learning a lot mroe about his earlier self. 7) Small short non-missions showing how Diamond Dogs and its obsession with destroying XOF and Cipher is having adverse consequences on members, their lives, etc.