Oftentimes a common complaint and/or debate in the series revolves around the overworld, or the lack thereof in many regards. As an example, I personally wasn't a fan of Ocarina of Time's overworld... specifically Hyrule Field, because it was empty for the most part and felt pointless. At the same time, it did add some sense of belief in terms of scale to the world you were saving. It helped make the world seem much bigger than it really was, which in turn gave greater purpose to your quest.
Now, this isn't an open-world conversation. Debates about linearity and hand-holding are for another time. Rather, this is strictly about the size of the overworld and what you feel they need to do with that overworld to make it feel whole. Personally, I don't really want it to be the size of Skyrim, but I wouldn't mind an overworld that was twice the size of Twilight Princess. However, with more size comes greater responsibility, and this is where we run into the "how to make this space interesting" aspect. Outside of placing more enemies, you could have more areas and collectibles to discover, combined with weather changes that make the world feel more alive and thus less boring to travel through.
Another aspect is obviously interesting and varied terrain, but realistic variations. Not "spring-time Hyrule Field" directly feeding into a dried up desert with a snow kingdom right next door. More believable transitions would really keep things fresh to me. I also wouldn't mind if we happened to have a boss battle that happened right there out in the open beyond the final fight. Volvagia breaks loose from Death Mountain and we catch it mid-field and try to take the dragon down. Something exciting like that, because it's unexpected and makes the wide expanse worthwhile. If you're in a fight with a giant beast in a big field while constantly moving, it really gives a sense of necessity to the space you travel in. How big do you want the overworld to be in Zelda U, and what would you do with that space to make it feel alive?