For me, it's a toss-up between:
A seamless, non-transportation overworld
and/or
A full-scale dual-overworld (more so than Skyward Sword's, which was really a main overworld, and then a separate 'overworld' used basically for fast-travel and to access the hub. I'm talking more like ALttP or OoA, except much larger and in 3D)
and/or
More questing. Like, JRPG amounts of questing. With meaningful rewards, like bunches of rupees (if the game gives a good reason to spend them), perhaps equipment upgrades or optional items; stuff like that. We could have more challenging foes and additional dungeons. This would also allow for an easy way to develop characters and have sub-stories outside the main plot. All good things if you ask me.
As for most wanted? I'd really, really like all three of these things, but if I could have only one, I guess I'd go with the questing. So many possibilities there.