This is a cool-sounding mechanic, but I don't think it's one that would really fit for this game. It's already really easy to miss stuff in the game thanks to the sheer amount of stuff to interact with in the world (I'm someone who thinks the amount of interactible stuff should be reduced by a bit) and if companions can go out and interact with stuff on their own, that could make keeping track of where there is and isn't stuff to find a real pain. Plus it sounds like you want characters to actually wander off and explore on their own during gameplay, which sounds like it would be incredibly difficult to manage with the current control scheme. Things like traps and potentially getting into combat or just losing track of your companion when you need them for some kind of skill check all make for compelling reasons not to include this feature.
What you've described is, in my opinion, a cool mechanic that should not be in BG3. It sounds like the sort of thing that needs to be conceived of as part of a game from the beginning so that everything else within the game can take it into a account. Adding something like it this far in, when clearly so much has been designed, seems as though it's just asking for trouble. But you're right that it's got a lot of potential in another game.
