Ok. Let's take one reaction preset at a time. Attack of Opportunity is the primary reaction everyone has.
AOO presets - All Enemies (Default); Selected Target(s); Off
Basically, players will standardly want to have AOO trigger every time any enemy leaves melee without disengage. However, there may be times when they don't want to waste it on certain enemies. Example: Don't want it to trigger on goblin. Want to save it for the bugbear who might deal more damage to my mage if he gets past me. So, Selected Target(s) would allow the player to, during their turn, set AOO and then pick only those targets they want it to work on.
Keep in mind that most fights in BG3 have 4-6 enemies, so this really wouldn't take that much time even if you had to do it for 4 or even 6 party members. And again, how often would you really NOT accept an AOO against an enemy who triggers it?
Note: The game would need to also set priority on AOO, maybe based on initiative. If 2 or more PCs are capable of attacking a target with AOO, they should attack 1 at a time instead of having them potentially waste their Reaction. In other words, if PC 1 kills the target with their Reaction, PC 2 wouldn't waste theirs attempting to kill a target that is already dead.
But what if PC 1 has no other enemies near them, but PC 2 does? Priority would need to be set to PC 1 to use their Reaction leaving PC 2 able to still use theirs if another enemy attempts to run.
AOO can be combined with special abilities like Smite and Battlemaster maneuvers and Sneak Attack. So, I suggest - players who love Larian Chain System are gonna live this - the ability to chain Reactions to Abilities. Drag the ability button and chain it to AOO (or right click on AOO button and a list of chainable special abilities appears... Or something similar). So, do you want Smite to trigger with AOO? Drag it and drop it on the AOO Reaction button on the UI. Popup appears asking to set preset:
Chained Special Abilities Presets - All Enemies (Default); Enemies HP > X (you set the HP amount the enemy has that would trigger); Selected Target(s); Off
So, for Smite, I set the HP to 10. Only enemies with 11 or more Current HP would trigger the use of Smite. If an enemy only has 10 or less, my PC would do a regular attack with no Smite.
Rogue wants Sneak Attack to apply to AOO? Drags, drops Sneak Attack on AOO. Sets preset to 12. If enemy has 13 or more Current HP, Rogue would use Sneak Attack as a Reaction if he hit.
Battlemaster wants to use Trip as AOO. Drags and drops. Wants it to trigger on any enemy. Sets preset to 0. Now, enemy attempts to get past Battlemaster. She attacks automatically and uses trip, every time. Battlemaster has suddenly become an effective enemy stopper without the need to pause combat every time to ask if she wants to use Trip.
What about Smite Level 2 or Level 3 or higher? As PC gains higher levels in Smite, the preset allows them to set each Level's interval. Level 1 sets at, let's say 10. Player sets Level 2 at 20. Player later sets Level 3 at 30. Game then knows if 10 or less, no Smite. If 11 to 20, Smite 1. If 21 to 30, Smite 2, etc.
For those times when, again, you want to just select which enemies would trigger the special abilities reaction, this preset is once again available.
Did I miss anything for AOO? Would this pretty much cover it? I'm fairly certain this would do the same thing as the prompts for 90ish% of all AOO scenarios. Thus, if this worked, it would make it so combat could still run smoothly and still give players close to perfect control.