Something like Skyrim, Fallout series, Cyberpunk 2077, Assassin's Creed, Deus Ex series.
Yeah, one may argue they do have bosses, but for the most part, they are left for the culmination of the game and most commonly there's only one, so most of it is nice.
Would love to have final bosses removed as well though - except for Skyrim, which is one of the rare cases the final boss is reasonable and lore-backed.
I cited them as examples of open world mostly without bosses that I had fun with, not some "pinnacle of game design".
I didn't specify the Fallout games, and I had fun with NV. Fallout 4 is actually not bad, too, despite all the negativity.
Like it or not, TES 5 is good and highly appraised; it became a phenomenon of its own. Yes, Morrowind and Oblivion are good games for their time (and Oblivion intro still gives me chills), but from a modern gamer POV, they are more nostalgic than great in today's terms. They were great hits in their time, however, and I appreciate their legacy. I did play parts of all games in TES series, starting from Arena, I have Morrowind still installed on my computer, and I'm not ignorant when I say Skyrim is good and is my absolute preference.