I would love for this game in DLC (or the sequel) to have Aloy gain the ability to order or instruct her overridden machines in some fashion.
Sometimes it can be frustrating to override a badass Ravager and then go attack a neighboring robot group while the Ravager sticks to his route and never joins in no matter how much I try and lure the enemies to them.
It would be super badass to be moving as Aloy and having a pair of Ravagers or Sawtooths just following her wherever she goes (within a certain radius as to not get too ridiculous).
Can't see this happening in DLC because it'd break the existing game's balance (such as it is, anyway) pretty hard, but it's clearly strong mechanical territory for a sequel to explore.
I think an even stronger override (i.e. getting you a permanent non-steed companion, or being able to ride any overridden machine maybe) would be seriously cool but would clearly need to be limited to one machine at a time and/or have a much longer cooldown.
Improved melee mechanics (I'm really partial to the idea of getting to do critical/stealth strike type damage if you execute a 'perfect dodge,' bullfighter style), substantially improved human AI (they should be scary because they're smarter than machines and excellent at teamwork), and an intensified focus on using harvested parts from machines (the weapons they drop are cool and there should be more of them, but I think being able to use a Stalker's cloaking module or a Shellwalker's shield until their batteries run out would be brilliant) seem like the way to go in my opinion, plus more in the way of environmental interactivity.
It's an immensely solid foundation for a series and a brilliant game and I hope Guerilla is hungry to keep improving it in leaps and bounds.