As far as this game I would love to see DLC about Rost or another character with the addition of a different weapon or two for that person. An example being that in one of the early arts there is a man with a pistol so maybe working of something like that. The best thing about the game is that you learn to get better over time and just have more tools to help you get it done so keep that aspect moving forward.
Now for sequel it can be a continuation of the same cast that we have learned to love or we could go the Assassins creed or Infamous: Second son route and bring in a new cast but tie it to the lore and back bones of the previous. The current mechanics are things I love and with that we can always expand but as stated before the good thing about this game is your tools so don't lose that. With the massive world we have been given we could have maybe a system where we can bring on a hunter or two and help hunt machines with the main character as well,think ordering them in a way MGS 5 does with ordering "Place traps here or wait for ambush". Maybe a village mechanic similar to how Fallout 4 does where it is nice but not a game breaker if you don't put time into it. I am fine with it being a single player experience with no form of Multiplayer as I feel that with the addition of such a thing could take away resources from the single player. But if it was a thing considered to be added make it a coop hunt kind of thing. Groups of 4 seem to be a good number but each person can have a customization character that they level up and upgrade as well as acquire perks. With that I was thinking a system on how Uncharted does its multiplayer. You have X amount of points on your load out and a weapon cost some and so do skills,that way you cant have one player doing all the work and you NEED the others to help you.
TLDR: Game is wonderful keep what is there and expand,don't complicate things. Everyone toss out ideas.