They could probably just replace sand with snow, advance the story 5 years and update all the enemies/weapons, and give us new Mother Base upgrades, and I'd probably still put 100+ hours into it just because the free roam and gunplay so fun and I love the Diamond Dogs/Mother Base aspect.
Could see that making sense, too. Basically 'Mother Base 2' the game. Light on story, few cutscenes. Mostly just new enemies to recruit, weapons to upgrade, DLCs to sell. Upgrade all the existing art assets... tweak Afghanistan to be a snow location; tweak Angola to be much more dense forested jungle. Gameplay and engine is already done. They could probably get a Platinum in there to make their own version of 'Diamond Dogs builds Mother Base 2.0' with not a tonne of actually new assets and just make almost a sort of MGS V expansion.