>level design
>feel unique and intentional, and nothing feels copy-pasted
Son, you should try to re-learn the definition of "level design". It's not about making it feel like anything, it's about communicating a goal to the player, and giving them fitting obstacles to keep the gameplay fresh. Interaction with the level is the only thing you do in open world games when you're not engaging in combat or whatever else.
Open world is synonymous with backtracking, as your path through the game will be across one, large map instead of multiple tiny maps. As a result, effective level design in an open world environment needs to work for all the possible situations. You can already see how this is near impossible, especially when you add in the player's ability to free-roam. With no goals to guide the player towards, you have no idea of where the lines should point to, or how the roads will intersect. The developers can only imagine specific, predetermined scenarios where the levels they designed shine the most. Adding in content, secrets, or collectibles won't do shit if the level doesn't work for every scenario. And in the case of turds like Assassin's Creed, Ubisoft decided to make the player only have a handful of interactions with the environment (running and climbing). There's always some "clever side routes" to climb faster or whatever, but it's always just a random area that you either happen upon or remember from last time. There's no meaningful interaction with the level, you're just using the shit that happens to be in front of you– the city might as well be randomly generated.
It's not much better in the case of GTA, where you have gameplay both on foot and in cars, but without deeper mechanics (or ways to interact with the level), the game becomes boring once you've memorized the entire map. You don't really care if you can take a shortcut through an alleyway, since you'll be able to lose the cops all the same. You don't care if there's a good sniping spot at the top of a building since there's no incentive to use your sniper rifle. You don't care if there's two or three bridges across the river since you'll just take the closest one to get to the bowling alley to play that dumb fucking minigame one more time. This isn't how things should be; games should let you interact with the level, that's the whole reason the level is there.
The best open world level design is Tony Hawk's Pro Skater. I don't specifically mean the "open world" entries to the series, despite their merit. Maybe this doesn't count, but since the streets of 3's Canada or the Hangar in 2 feel more alive than any square mile of any GTA or Elder Scrolls, there's at least something to be learned here. Each map in THPS has multiple, distinct routes with infinitely many branching paths to do a variety of tricks. The game knows its own mechanics, and what kind of things you'd be doing with the mechanics and with the level to score well. The ability to score high in THPS requires the player to both know his surroundings, and to know how to use these surroundings to maintain a combo. Missions in the level encourage you to explore the map, as well as prove your mastery of mechanics and knowledge of the level design. Having distinct visual clarity and thematic consistency amplifies the ability to remember level characteristics and aspects, making even bad players want to revisit areas after they've completed the missions.