Reactions are part of5E so i guess it will be in the game.
And work like this: on the enemies turn it uses fireball.
Your character has the option of using a reaction or similar like luck.
You get a message that you can choose to use your reaction/skill/feat.
You choose. Computer calculates outcome then it goes back to the normal turn order.
Other example: on your turn you miss.but you are a battlemaster and can choose to use superiority dice to turn a miss into a hit.
So before your turn is over you get the option.
In hard mode you could give players say 5 seconds to choose to use their optional reactions/feats etc.
