There isn't really anything about the game that I actively dislike, as in the mechanics or game play - I have no problem with the breaking weapons as it helps mix up the players approach and there's never really a shortage of weaponry on hand any way!
In terms of basic stuff I think some of the UI and the way the player interacts could be improved - switching gear is still a bit clunky, the cooking is repetitive if you're brewing up a big batch of stuff etc. But those are minor points.
Some big things for me that I think could improve an already amazing experience are:
I want a more involved story, things to care about, insights to hear about the people around me and that make finding new places more fun. At the moment its a bit - "ooh a new village, I wonder what fetch quests I can do for everyone for no real reason!"
Majora's Mask is still the high point for involved narrative and questing in the series, do more stuff like that please Nintendo!
I've spoken about it on here before but I think the risk/reward structure of the game could do with some tuning - there isn't much that the game can give you to make battling harder enemies in the wild worthwhile. Slogging through a bunch of high level enemies, wrecking your weapons and depleting your arrow stocks to be rewarded with a small number of rupees or another crap shield is not the best experience if it happens over and over again!
I think they need to double down on the slate powers as well and truly embrace the emergent gameplay and empowering the player to come up with their own solutions to problems. There is a good amount of this in the game but mainly on a small scale. Various "puzzles" in the world are still only solvable by the prescribed method, give the player their powers and let them work out a fun solution themselves!
Again this is something I've discussed at length before - exploration is the backbone of the game but gets nullified way too early I think. There is so much joy in the challenge of navigation at the beginning of the game. Balancing limits like the stamina bar and planning a route up a mountain, surviving in the cold or heat etc. where you may need to create a makeshift camp to warm up, cutting down trees to make bridges or makeshift rafts to cross a body of water. This was really great stuff in the early game, but very quickly climbing becomes trivial, heat and cold can be dealt with easily and any water you need to cross comes with a handy boat nearby. And failing all that just climb a hill and glide everywhere! Exploration is still great in the game, but it could have retained so much more excitement from keeping that extra level of challenge and requirement on the player to think, prepare and keep their wits about them!
Some more indoor locations would be fun too - the Castle was great and the Divine Beasts were too, but I think there was more that could be done. I don't think a return to traditional dungeons is necessarily the right answer, and I didn't miss them as much as some seemed to, but something along those lines would be cool.
So yeah, various things that I would love to see them build on for a sequel, but my goodness is the game already so great!