I've been toying with a few different ideas on how to resolve combat
Normally, you attack and you either hit or you miss. In a PbtA game when you attack you exchange damage, doing it well means you potentially hit harder and take less damage yourself, doing it poorly means you could take more damage or inflict none.
What if...? You attack, and one of three outcomes occurs: you hit and your opponent misses you, you and your opponent hit each other, you are hit by your opponent.
The traditional calculation in D&D is: Class attack bonus + ability modifier + 1d20 versus 20 - armor class
But if I'm changing the rules to be level-less and armor reduces damage rather than makes it harder to be hit, then...
Ability score + 1d20 versus 20 (too easy, probably good to experiment with at some point though)
Ability score + 1d20 versus 30 (not sure, experiment!)
Ability score + 2d6 versus 20 (hard difficulty, some rolls will just never be outright successes though)
Ability modifier + 3d6 versus 15 (moderate to hard, the curve is shallower and more predictable than a 2d6 which might lead to less fun)
Ability modifier + 1d20 versus 20 (very difficult, wilder results but potential for critical successes and failures!)