Oh, easy peasy. Let’s say a monster casts burning hands.
DM: “The caster holds his hands with his thumbs together and his fingers outstretched, and a thin sheet of flames erupts from his fingertips. Alyssa and Bob, you’re both in the affected area. What do you do?
Alyssa: I try to roll out of the way of the flames before they hit me!
DM: Ok, make a DC 12 Dexterity saving throw. On a success you’ll only get singed for half damage, and either way you can use your reaction to move to a space outside the cone.
Alyssa: rolls Damn, I got a 9.
DM: Your reflexes aren’t quite quick enough. You can move out of the cone, but you’ll still take rolls 7 damage. Bob, what are you doing?
Bob: I’m not very nimble... I’m just going to grit my teeth and try to tough it out.
DM: You’ll take a direct hit from the flames, so you’ll be taking the full 7 if you do that. But if you pass a DC 12 Constitution save you can keep it together enough to make an attack against him with your reaction.
Bob: rolls 15!
DM: Awesome, make that attack roll.
Bob: Can I try to shove him prone instead of doing damage?
DM: Absolutely, let’s see that Strength check. The caster got a... rolls 11.
Bob: rolls 18!
I agree it's easy (ish?) for the DM to improvise in cases like this. What I think is hardeis to create a set of simple, clean, generalized rules that will work across a broad range of spells, without requiring a block of text for every spell.