#1 hopefully a new map in whatever the next game is
In BOTW I spent months exploring the world, eventually getting every shrine, just enjoying the beautiful landscapes. BOTW definitely had some detriments but the exploration of the world eclipsed those down sides. I remember my starting experiences well. The initial release trailers, E3, looking out from the great plateau at this giant world. Eager to get out an explore. My hero's journey touches most parts of the map.
For me, TOTK doesn't have that same spark to explore. I even completed the game without unlocking all the map first, despite taking my time. But I got to the point where I felt the next step was to beat ganondorf. In comparison to BOTW I basically feel like I'm done with the game now.
#2
I think they can continue to improve the narrative even in an open world experience. I think you can see the primitive pieces in TOTKs quests from purah which could be expanded further for more narrative and maybe even make a more linear experience in tackling dungeons.
I do not think they will go back to linear adventures. I hope that the experience that they have gained in TOTK will allow them to integrate linear sections into open world zeldas. The benefits of linear sections being the ability to teach something and then ramp up the difficulty in applying it.
#3 better temple
The temples in TOTK were
I understand that part of the 'temple' now is just about unlocking it but I feel like we had that in twilight princess + a dungeon.
There are a lot of possibilities I think for a semi-open temple.
Imagine having even four temples which were similar to the great plateau - areas designed to sandbox and let you muck about solving puzzles but with constraints.
Or just a continued evolution of the designs from BOTW -> TOTK, but have more terminals, branching paths and puzzles and unique designed combat challenges. Add in a new ability which you use to solve the dungeon and need it to get into the next one?
TLDR
Needs new map.
Likely to stay open world
Hopefully keep evolving open world and add in more linear or more constrained areas including better temples