Like Hellish Rebuke. The problem with Hellish rebuke, in my opinion is not that you have to cast it before. It's that I have zero control on the target. The game could just ask me to target lets say up to 5 ennemies (or click the "all ennemies icon") before casting the spell. Lets say I choose 5 targets, the first one in the initiative order among those that's attacking me I've selected burns.
If I can "recast it for free" at each turn, I have way more control on the triggers than currently for the price of an additional things during our turns on top of not adding flow breaking questions during rounds.
This sounds like good idea. :3 I like it!
I'm mostly sure there could have a good solution for all reactions in the game. In my opinion it could be a good compromise between combat flow and control. Characters turns are a bit longer and the player has a bit more things to spend than speed + action + bonus action, which seems interresting to me.
Not something you have to do each turn but you can depending how things are going.
It could become a specific feature to use like spells and class features. You set up actively your reactions during your turns rather than writting conditions in a book or answering the constant questions of a popup.