Breath of the Wild was an amazing new direction for the Zelda series and a more than competent open-world game in its own right. Like many open-world games, though, it suffered from a huge world with not exactly all that much to do in it.
Don't get me wrong: I explored every inch of it, twice. However, after a while, or depending on the situation, climbing up another mountain or venturing off the beaten track isn't quite as appealing, since you know there's probably just a Korok Seed to be found at the end of the journey.
In contrast, traveling over a mountain range (and not being spoiled by a guide) only to find a new village, a massive monster, or a group of travelers gives a thrill like no other. It just doesn't happen enough, but it's the kind of thrill the sequel needs to have.
A big part of making the world more engaging like that is, naturally, adding more places to see. Towns are the first thing that come to mind, but the world did just get wiped out a century ago, so bringing in a bunch of new towns probably wouldn't make sense. However, the seeds of new settlements would make sense. So would having more collectibles that make a bigger difference on gameplay the more you collect (unlike Hestu's poo gift for finding all the Korok seeds).
More mini-games wouldn't be unwelcome either, but one other thing that was definitely missed in Breath of the Wild is the trading game. Adding that to a vast, open world might be the equivalent of finding that one neighbor who wants their tape back in the original Animal Crossing (read: very frustrating), but there's no denying trying to find out what item is needed and who wants it would add an extra bit of challenge and give a reason for exploring everywhere.