-Alteration spell that repairs armor/weapons. Call it 'Mending' or something.
-Sneak attack criticals for magic. Maybe a if possible a way to emulate the Skyrim silent casting perks at certain skill ranges for Illusion and/or Sneak. They don't have to be monster bonuses. Preferably fair and balanced.
-Oblivion style craftable poisons and reworking monster poison resistances to give it more utility (which I think is somewhat poor in the base game), casting cost rebalance for poison destruction spells.
-Fatigue threshold mod for human NPCs that will make them yield when beaten instead of always choosing Valhalla like absolute lunatics. Perhaps scaled by some factor of willpower/endurance/level?
-Enemy level scaling that gives NPCs boosts to their relevant skills only to prevent Oblivion style problems like HP bloat and/or Glass Armor Bandits Daedric Marauders.
I think this would subtly make your opponents (at least the human ones) subtly deadlier and heartier
without becoming absurd minibosses that force you to minmax.
The hope is that this will make finding 'boring' caves with peasant smugglers or rogue mages scale like ancestral tombs without being too heavy handed.
-Fixing higher level skeleton fatigue so that HTH players aren't having to spend 3 minutes apiece on them at high levels of HTH skill/strength.
It's not challenge, it's a hidden HP bloat factor that only HTH players notice.