The idea of peripheral vision can be very fun in dungeons, all party behind an elite mob!
Regarding the distance I remember a game, Comandos, where the npcs has a peripheral vision of two types, one that automatically generates aggro, an another than doesnt generate if you were stretched out on the ground. Using the same mechanic you can do it with a lot of things, for example, you are in the "light" agro zone BUUUUT you are inside a bush (water, or any other kind of similar environment), so you dont aggro him. Just if you go out of bush, or get in the "hard" agro zone.
Peripheral vision can be very fun and realistic in dungeons, where the player and their party have to sneak behind an elite mob or avoid being detected by other enemies. I think that this would make the game more thrilling and strategic, as the player would have to plan their moves carefully and use their environment to their advantage.