First off, the same world doesn't mean the same map. Most Zelda games have featured different versions of Link and Zelda, so same world means another adventure of the same Link and Zelda. Whether Breath of the Wild's map is used as the majority, only partially or perhaps not at all is up in the air. The only thing I feel sure about is that BotW2 won't just be a slightly altered version of BotW's map.
What I want to see change:
1. More than one dungeon this time. BotW had only Hyrule Castle.
2. A complete set of unbreakable weapons (so one each of sword, great sword, club, lance, boomerang) that can be upgraded over time. The idea is that breakable weapons still allow for the same kind of freedom as BotW, but eventually the unbreakable weapons can and will outpace (almost) all breakable weapons. The unbreakable weapons will require certain numbers of unique ore which in turn will solve another shortcoming of BotW, which was not enough valuable items scattered across the world. Additionally, this helps to have sidequests with better rewards.
3. Instead of making almost the entire world subject to a universal difficulty that raises over time, I'd like to see a variety of unique locations like enemy fortresses being locked at a certain difficulty. This makes for a more interesting world because what BotW faced was a world that felt ultimately very similar regardless of which direction you took. Also, let those locations have some meatier music than the plain overworld.
4. Equipment that gets around all sorts of environmental disadvantages. BotW didn't have equipment to get around rain while climbing. Alternatively, and probably preferable, a bunch of equipment traits of BotW should be directly integrated in Link as a character, so his climbing skills could eventually get good enough for any kind of weather without the need to put certain equipment on.
5. More enemy variety, including bosses.
Things that should remain the same:
1. Shrines as warp points that have challenges inside of them. Of course the test of strengths need to have a lot more variety this time around.
2. The ability to glide. It's one of BotW features that aren't appreciated enough. If you climb up to high spots in other open world games in order to get a good view and find points of interest, you have to find a way down afterwards. This waste of time doesn't exist in BotW, so climbing up a mountain comes with the additional benefit of reduced travel time to points of interest you spot.