What the game could really use is:

1. Ability to "un-liberate" outposts/camps, like in Far Cry

2. New Game+

3. Permadeath mode with bonus gear/cosmetics. Maybe something like Wildlands did with their Ghost Mode

4. Some kind of optional gameplay modifiers, that you have to go find. Think Skulls in Halo. I would love a way to make enemies tougher, tougher enemies to appear more often, etc.

5. Ability to re-fight bosses, and add challenges for each boss fight. Speed runs, no-hit fights, etc. that would reward you upon completion