The only game I really enjoyed exploring every single nook and cranny was TW3. It felt massive. It had some fairly empty areas to be sure…but that was made up for by music, weather, terrain (that you actually had to navigate over with Roach), and gorgeous skies. And hairworks…for parking Geralt on a cliff and watching the clouds role by in style.

The atmosphere was so great that I often explored with no hud and no music just to immerse myself in exploration.
Another great game for that was Skyrim. But both of these games are as close to true open world as one can get. And they are single player deals.
Dragon Age Inquisition, despite its many story and gameplay faults, actually found a good balance with open world-like areas combined with hubs. Works well for party based rpgs imho. I think that could work well in a game like this to give the weighty sense of scale and place and add some filler npcs — tho I am now ducking to avoid eggs for saying that.
Also…just fired up DOS 2 for the first time ever in my boredom while waiting for next BG3 major patch. Man…that intro compared to BG3? Shocking how similar it was. Kinda miffed me.