And, you know, I was giving it more thought too. Not sure why it might be more difficult for multi-player now that I think about it. It's turn based. You COULD allow players to pick their reactions similarly to Solasta. Enemy attacks with magic missiles, game pops up a window asking if you want to cast Shield. You click Yes or No. Game resumes. Enemy casts the spell. If you hit Yes, missiles hit you and nothing happens. If you hit No, missiles hit you and do normal damage. Wouldn't drag the game pace down that much. It works for Solasta. I actually like that feature in that game.
Same with deciding if you WANT to use your reaction on a particular enemy as AOO. Sometimes you are surrounded by more than one enemy. You may want to use your Reaction to hit an almost dead enemy who tries to flee, but one who has more health moves first. In BG3 now, you would auto-react to hit the bigger enemy and thus the little one gets away and shoots you without even triggering AOO. However, if I was given a choice, I might not use my Reaction to hit the first guy but to hit the second.
But you are right. Making it another Option in the Options menu would allow players to play whatever way they like. The only issue that might come up from that, though, is that Larian has to do a lot more coding to implement manual reactions just so people have the option to do it. If we keep making them add more features as options just so everyone can play the game the way they want, the game might wind up being 300 gb in size before we're all done. Not saying they shouldn't and that it wouldn't be good. Just saying that I could see that as an issue with the idea of making it an option.
I personally think manual reactions would be awesome. I'd love to see it.