While you’re asleep, you should recover HP and energy faster than normal, possibly even at the 1 per turn rate that the extrinsics give you.

Druids’ mistletoe requirement for maintaining good energy regeneration should scale up as they gain more levels rather than being a fixed amount, because as soon as you have that fixed amount there’s no reason to find any more.

One proposed formula is: energy regeneration is halved if you possess less mistletoe than your XL, regular if you have mistletoe equal to your XL, and increased if you have more mistletoe than your XL, possibly with some exception for very low level Druids.

One of the dragon armors should confer energy regeneration as its primary or secondary extrinsic. The problem is there aren’t any obvious candidates which would be appropriately flavorful, other than gray dragon armor, and that is unsuitable because it would be overpowered.

Energy recovery rate should be leveraged more as a role differentiator - basically, roles that are intended to cast frequently should regenerate power faster than roles that aren’t.

One way to do this is tie energy recovery strongly to Wisdom, and then bias starting stats so that caster roles usually have high Wisdom.

Standing on a magic trap’s space causes your magic energy to regenerate faster than normal; not incredibly so as if you had the energy regeneration extrinsic, but a noticeable boost.

Change the HP and/or Pw regeneration formulas (natural regeneration, not from external sources) so that it regenerates faster in the early game but more slowly in the late game.

This fixes existing balance problems – that the hero doesn’t regain them fast enough early on, and by later on they regain them so fast it trivializes problems – but would be very strange to justify flavor wise.

Being polymorphed into a spellcasting monster boosts your energy regeneration rate proportional to the base level of the monster you are.

Make the Mitre of Holiness invoke effect give temporary intrinsic energy regeneration rather than a single huge boost of power.

Chargeable ring (or amulet, if it’s feasible to make chargeable amulets) of energy regeneration.

An early-game body armor piece that increases Pw regeneration.

Resting on a bench regains Pw as well as HP.

Energy regeneration should cause you to hunger faster.