I think most of us here can agree that Horizon: Zero Dawn was an amazing game. That being said, there's certainly lots of room for growth in the next installment.
Machines were undeniably awesome, but human enemies were lacking in terms of variety. Their AI certainly didn't help things. The humans weren't necessarily un-fun to fight, just not as cool as the machines. What I'd really like to see is more types of human enemies. How about humans riding on machine-back? Maybe humans could wear armor that gets shot off or damaged just like machines. How about humans with armor that resists elemental damage?
I would love to see deeper, more challenging puzzles in underground areas. I understand that this is a combat and story focused game, but it would be a cool thing to have, nonetheless.
Take a page from Zelda and let us climb nearly anything we want (within limits, of course). I eventually grew to enjoy the climbing paths with all of their jumps, ziplines, and tightropes, but they made everything feel very scripted. I can't tell you how many times I wanted Aloy to grab a ledge that by all means she should have been able to.
I think a co-op mode, where you and a buddy team up to take down machines, would be nice. I don't necessarily want Horizon to have a co-op spinoff (think Triforce Heroes, Metal Gear Survive, etc.), but a small multiplayer mode would be cool.
Honestly, all that Guerrilla really has to do at the end of the day is just give us more. More machines, more weapons, more stories, and more Aloy. But these would certainly sweeten the deal.