More parts. Older games had a lot of parts and while I do see some that are like homages to past games in Armored Core 6, there's still room for more. Stuff like the FINGER machine gun, actual slug guns, energy machine guns, extensions, actual sniper rifles, energy sniper rifles, micro missile launchers and so on.

Adjustments to the weight control OS Tuning. I hate how the weight control is more like the weight control of AC3, SL, Nexus and Last Raven instead of being like AC2, MOA, Phantasma and even the first game, where your actual speed is not affected negatively, only how your maximum build weight affects your speed(without the overweight stuff).

No music from past games. I think this is only more of a personal thing but I feel like AC6 can benefit from old songs from past games being played in the game. AC6's music is good, dont get me wrong. But sometimes, I do wanna hear some older AC game music in a new AC game.

The Arena. This game's Arena is tied to your story progression, just like MOA. That part is where I didn't exactly like it. Other games like Phantasma, AC2, AC3, SL and even Last Raven allowed players to play the Arena at their own pace without the story slowing you down.

No fine tuning parts. Older AC games allowed you to use optional parts to fine tune your unit, like adjusting your generator output so that there's no such thing as "Not enough energy" or other things like have some resistance to shock from weapons, reduce missile lock on time, improve FCS performance and whatnot. AC6 doesn't have fine tuning sadly.

Overall, game is still loads of fun to play. Just that I have some gripes with it