Metal Gear Solid 6 is a game that, truthfully, will never be released. The falling out between Hideo Kojima and Konami, along with the pretty closed off nature of Metal Gear Solid's story, make a sixth entry seem highly unlikely.
As far as influential game franchises go, Metal Gear Solid is one of the most impactful. The brain child of Hideo Kojima, Metal Gear Solid's combination of stealth-action gameplay and immersive storytelling blew fans away in the classic 1998 PS1 title, Metal Gear Solid. For another 17 years, Kojima and Konami continued the Metal Gear Solid franchise, taking the story into the near future with nanomachines in Metal Gear Solid 4: Guns of the Patriots, and to the Cold War in Metal Gear Solid 3: Snake Eater. Any chance of another sequel died with the release of Metal Gear Solid 5: The Phantom Pain, though.
No Hideo Kojima, No Metal Gear Solid 6
Those who have played the The Phantom Pain know it is an unfinished game. From promises made by Hideo Kojima about MGS5, to files exorcised from the game itself, there is evidence of a lot of content that never made it into the game. One of the most infamous examples is Episode 51, which explains what happens after Eli steals the Metal Gear at the end of the game, rather than the strange, dismissive cliffhanger that is currently at the end. Due to the game's troubled development, stemming from Kojima and Konami's relationship, those pieces of the game were not featured.
Well in advance of The Phantom Pain's release were rumors that Kojima was leaving Konami. Eventually, rumors became action, as Konami removed Kojima's name from the marketing for The Phantom Pain and would eventually deny him from leaving Japan to attend The Game Awards in 2015. Kojima has never gone on the record to explain what precisely happened, but it is clear from the development of The Phantom Pain, and the later cancelled Silent Hills, that whatever happened between the two parties was irreparable. Konami would try making its own Metal Gear game with 2018's Metal Gear Survive, a zombie-survival game running on The Phantom Pain's engine. However, after reviewing and selling poorly, it was the nail in Metal Gear's coffin.
No Story for Metal Gear Solid 6 to Tell
Metal Gear Solid's story has a clear beginning, middle, and end, even if it's a little messy getting there. At the end of Guns of the Patriots, every Snake gets their conclusion: Solid Snake, his battle with his brother Liquid, and his search for their father, Naked Snake, is poetically completed - with Solid Snake likely dying of old age off-screen. In Kojima's eyes, the story he wanted to tell went as far forward as it could, explaining why he spent the last decade at Konami filling in the blanks between Snake Eater and the original Metal Gear through games like Peace Walker and The Phantom Pain.
As for Metal Gear Rising: Revengeance, although its overall plot was developed by Kojima and takes place after the events of Guns of the Patriots, it is not Metal Gear Solid 6. Without controlling one of the Snakes, and with minimal connection to original franchise, it is essentially its own game. The only Metal Gear Solid 6 that would make sense is wrapping up the events between The Phantom Pain and the first Metal Gear, but it seems like that is never going to happen.
Metal Gear Solid 6 is not known to be in development.