This is something I thought of last night. I didn't play any 3e or 4e; had a 20 year hiatus from 2e and got back into 5e with the playtest. So I don't know if the spell Witchbolt existed prior to now. But I find it especially awesome since it really recalls to me the kind of magic duals made so popular by movies --- two wizards facing off with rays of power, until one finally overwhelms the other. Honestly, I can't think of any more popular "magic combat" trope than this. And D&D never really supported it. But seeing Witchbolt, and seeing how they now made Shield a reaction, it occurred to me that Witchbolt could be an action -or- a reaction. You can cast Witchbolt as a reaction when another wizard casts Witchbolt at you --- this initiates a contest (Either Arcane vs. Arcane or probably just Spellcasting Ability Score vs. Spellcasting Ability Score). It can be a simple contest, with the winner canceling the other's spell and dealing damage. Or, more complicated but more interesting: one could really draw this out... with one wizard having to defeat the other by a certain margin before defeating them, canceling their spell, and doing damage, and with a stalemate until that happens. Then you get the full cinematic effect. Is it likely to happen frequently? Maybe not. Or maybe this kind of thing is built into the culture of high-magic societies.
In principle, it's super cool (to me), and something the new concentration/reaction combination allows for, which I don't think was workable in the early editions.
In principle, it's super cool (to me), and something the new concentration/reaction combination allows for, which I don't think was workable in the early editions.