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.