Ok, I'm playing Dark Souls 3 now, so here is an idea about one-button Dark Souls.
You have one button - Attack. Attack takes some time (~1sec), during which you are vulnerable. If during this time enemy does not attack too, you score and enemy loses HP. If he DOES attack, then your attack is interrupted, and YOU are damaged. If you are attacked when not attacking yourself, you automatically block, receiving no damage.
Enemy attacks in pattern - same one each time, or he can have several patterns that he uses back-to-back, choosing randomly. Patterns can be generated procedurally.
Gameplay is basically learning the patterns and finding openings in them.
Graphics can be very simple, or there can be no graphics at all - it can be all done with just sounds.