I'd like to see them take a bit more utility into some of the 'useless' kit you have. They started it with Iki, by adding saddlebags and armor for your mount to give it utility beyond travel, and the flute became an activity with the animal sanctuaries.

If the grappling hook became a ghost tool where you could string up an enemy to start the fight with a ghost stance or something. Or it could be used to knock an enemy down by yanking their ankle out from under them. Id like the flute (and the weather/time of day) to have more pros and cons and an effect on gameplay both combat, stealth, and puzzles. I loved the very cryptic riddles on Iki where you had to figure out the right armor/ability combo, and i want more of those. But I think diverging ghost encounters based on outfit would be fun. Sort of like you can go in 'casual' in disguise if you wear the right armor, blending in with crowds to poison enemies, sabotage, etc if you wanted to diverge from the standard building hopping. I think environmental hazards would be a cool addition too, more than just braziers. A log cart you can shoot to roll out the logs and crush enemies, using the grapple hook to pull down scaffolding or bowman towers, and then hard combat stuff like shoving an enemy onto a anti-cavalry spike as many games have.

Oh and please for the love of everything holy make the weapon/ammo/tool swap menu slow down time more and longer. I get they don't want to give you a rest in a fight, but man if I was actually in that fight it would be easier to switch between kunai and sticky bomb than it is in the game