Gamma World / 4ed Defenses adjustment [4ed]

I'm going to be running a Gamma World one-shot and I was hoping for feedback on a rules adjustment I'd like to make.If you're not familiar with GW, the core rules are fundamentally the same as 4th edition D&D. Monsters, in fact, are totally cross-compatible.I'm thinking of changing the characters' static defenses to d20 rolls, and monster attack rolls to static DCs. Mechanically it's as simple as subtracting 10 from the defense values and adding a d20 roll, and adding 10 to monster attacks and omitting the d20 roll. In game, the result is that I would tell the player that a monster is attacking them, ask them to make an AC, Endurance, Reflex, or Will save, and then roll damage if they fail.My overall goal here is to move more of the dice rolling to the player's side of the table. As a DM I have plenty to keep track of without having to do math and rummage for dice.Can you think of anything wrong with this system? Complications I'm not considering? via /r/DnD http://ift.tt/2tB5TZ6