The keys to making a good open world are:
1. No matter where you go, there should be things to do.
2. The things to do should be things worth doing.

This is the conclusion I came to after playing Zelda: Breath of the Wild (and, more recently, Genshin Impact, which cribs a lot of the best parts of BotW). There's not a lot of point in having a big, beautiful map with all kinds of interesting features if there's nothing to actually DO at those features. Walking around on foot, you shouldn't have to walk more than a minute or two before you find something worth doing; a pack of mobs guarding a treasure chest with useful things inside, for instance, or the entrance to a micro dungeon, or an NPC in distress with a mini-quest to complete.

FATEs and Hunt monsters are a decent start, but not nearly enough - and the places that don't have FATEs or Hunts have a handful of mobs to kill, at best, and killing mobs is seldom worth doing.

Admittedly, though, the effort needed to make the overworld worthwhile would be pretty intense. They'd have to scale back a ton on the instanced content in order to have enough manpower for the job, and they've already scaled back on the instanceed combat quite a bit as it is!