transtemporal
Explorer
It has to be noted that the final version of 5e has effectively replaced Strength, Intelligence and Charisma saving throws with ability checks. You already roll an Investigation check against many illusions for example. So @Rune is not completely off with his idea.
Ah ok, I haven't totally caught up with the latest version yet.
I don't mind maybe burning your reaction or bonus action for the round to get a boost to defenses. I can see narratively how you could justify acrobatics, athletics or insight as a reaction or a bonus action. Medicine might be a bit harder.
Just keep in mind that this would make monster powers less powerful.
In addition, you would have to think how this applies to named NPCs including named enemies. Can they use it?