Hello,
Before you toast me completely for even mentioning PvP in a single player D&D game such as BG3 I would like to describe to you what happened and why I actually find it fun.
So, me and a friend were playing co-op through EA and one day while sitting for a while in the camp I found that we can actually fight with each other. I was a ranger class and he was a druid. Holding left Ctrl key I was able to attack him and it worked pretty damn good to be honest. We fought for a while and then when his druid died (several times, after resurrection) he switched to Lae'zel (managed to kill me few times). It was really fun. What ruined it though was that my friend discovered that when you attack using "left mouse click -> attack" the character you're controlling never misses and hits even from a large distance which is some kind of a bug I guess.
I honestly am not 100% sure whether should I go in that direction, but as I said it was really fun and was working surprisingly well as mechanics and everything, before discovering the bug. What I say is - try it when/if you are playing with friends and see. Personally I won't mind seeing such thing in the game (bugs free of course) as so simple PvP would be something not very hard to implement. Just an option to save your main character separately, some camp layout type arenas and through the multiplayer to find a server/opponents. Doesn't sound like much does it?
Tell me what you think or whether you have tried it.