I could see reactions having a 3-way toggle:
1) Never do this.
2) Do this whenever possible.
3) When the opportunity to do this comes up, ask me.
So characters that only have opportunity attacks as a reaction can leave it on option 2 most of the time (unless they are trying to catch one particular enemy and don't want to waste their reaction on the wrong one), so it just does the reaction without bothering you too much. Once you get multiple options, switch some/all of them over to 3. This way you can have it ask you as much/little as you like and it fits the current interface decently well.
