Add more sources of nausea, particularly in the late game, because it’s an underutilized debuff that only really is relevant early. Proposed sources:

• Certain types of demons or the presence of certain demon lords such as Baalzebub or Juiblex.
• When you are hit with a disease attack but for some reason don’t get deathly sick, or are already deathly sick.
• Stepping over a rotting corpse.
• Some smallish chance every turn you are in a cockatrice nest.

Most of these increased sources of nausea should be subjected to some sort of (low-chance) Constitution saving throw so that characters with better Constitution have an advantage.