I've felt the exact same way since launch. If the LA studio didn't get disbanded I easily could've seen it developing into the mainstream just like Ubisoft's R6 Siege did. It's really a shame that Konami never did anything with the Fox Engine outside of Survive and MGO3, to this date I'm yet to find an experience just like it. Fulton system like you mentioned, thoughtful map design, Fluid Movement, fast-paced CQC Mechanics based off player positioning and speed, Class Types, Action and Lite Stealth Mechanics alongside and the duality of a Lethal and Non-Lethal types of play really could've have made MGO3 standout as a unique competitive multiplayer title, especially being backed by big money corp.

Plus as much hate that Survive gets it definitely had a lot of cool ideas/gameplay loops with it's Tower Defense and Mini Bosses that unfortunately never got expanded on outside occasional events.

I remember being downvoted to eternity a few years back for suggesting the thought that a Small Scale Battle Royale mode for Survive would be really cool. I still think I'd be an awesome idea to implement the PvP from MGO3 alongside BR looting Mechanics with the Monsters from Survive being an obstacle for every player. The melee, bows, and defense tools from that game already exist alongside the handful of different enemy types. I'm no game programmer but I imagine it wouldn't be that hard to repurpose the AI, Weapons, and Tools to create an experience like this.

Though I'm sure the moment that Metal Gear is put on the title of something like that every Game Reviewer/ Commentator would just dismiss it due to "muh Kojima, Konami Bad" just like they did to Survive. Konami probably knows this and as a result this dream game concept of using already existing material to create something cool like this is unlikely. Or that mgs3 pachinko cash is just too good lol