I'm so excited for BotW2. Speculating is always fun!
I've been mulling over and thinking a lot about this game non-stop since it got announced. One thing that really stuck in my head is the quote that they're taking inspiration from Red Dead Redemption 2. Granted, it's a quote that one perhaps shouldn't read too much into, but it really made me excited. RDR2 is possibly my second favorite open world game after BotW, so I've really thought a lot about what features or inspirations from RDR2 that would be awesome to see in BotW2.
One thing that really hit me, and that they've touched a bit on in AoC, is to have several key NPC's - maybe the descended champions even? - have multiple questlines, in line with how RDR2 does it. Like, Tarrey Town-tier quests, but maybe 5 or 6 of them, just like RDR2 had Lenny, Sadie etc. Just opening the adventure log, and see, like, "Riju needs to speak with you" (not in that wording of course) and then head out there to participate in some cool, inventive mission that results in a neat reward. And then, after a while, the next Riju quest opens up etc. I think side quests in general is one of the things I hope to see the most improvement of in BotW2.
I really liked in RDR2 how you really had to take care of your weapons. It made them feel important, and gave them weight. If BotW2 keeps the degradation, I could imagine them fleshing the system and idea out a bit more by incorporating some kind of system where you could extend weapon's durability by taking care of it. Several people have mentioned blacksmiths, and I'm really into that idea, how you could maybe spend some gems and/or rupees on a weapon to extend it's durability and maybe even damage output. And a meter perhaps, that shows how much durability is left, I think that could make for a quite good QoL improvement.
I've also thought about some kind of wanted system. I didn't like how BotW kind of gated the progression to certain provinces - Akkala and Hebra had black/silver enemies from the start almost, and the game kind of ushered you into Tabantha/Lanayru/Gerudo/Eldin (the story-specific provinces) and it was quite discouraging for someone who wanted to go to Akkala right away from the plateau. I could imagine that every area starts out with enemies that you could take on, and once you've defeated enough enemies in an area, the enemies scale up in that area. (But they would still also scale up according to how many hearts/stamina you have, so you can't just stock up and then powerhouse your way through every area just because you haven't picked any fight yet.)
And finally, a weapon wheel instead of what BotW has.
Gosh, I can't wait for this game.