Having just recently played (and now finished) Breath of the Wild, if we're going for open world Halo third-person "RPG", I'd love to see a game in the style of Breath of the Wild. The limited inventory system and weapons that are frequently replaced, upgradable armor system, armor abilities, roaming around a lush world and restoring ancient Guardian Forerunner technology, and undergoing trials constructed by the ancients to protect their technology... could be awesome fit for Halo.

Hell, I've already said in another post that the enemies in Breath of the Wild are basically the same holy trinity of enemy design that the Covenant are - small cowardly swarmy guy, medium-sized agile lizard dude, and the bigger dude who bosses the swarm dude around and is a formidable warrior.

Bigger difference is that I don't see a Spartan as the type to be picking berries, cooking meats, and mining ores, but you could probably replace those activities with some sort of lore-appropriate collectibles.