If mana recovered slowly enough, and mages could pass it on freely between them, the ephemeral force would become a valuable trading commodity on its own.
What if a mage passes his powers on to his firstborn son, and a female mage to her first daughter, leaving the parent POWERLESS? A plain mortal, a sorcerer no longer, all wizardly might gone?
Or: what if a female wizard recieves the power of a mage who sires a child with her? Could lead to promiscuous witches and overly careful mages.
What if cancer is not an ailment if one has a will powerful enough, and knows what do do. What if one using the right techniques could control it and use it to become immortal, or to grow into something else?
Pick up a pathology/microbiology book, and add the diseases/syndroms to your game. Your players will freak out.
Coughed at? Diphteria. or tuberculosis?
Drunk from a muddy stream? Or just walked through? Well, let us introduce you to schistosomiasis...
Hit by a stream of energy? Too moch mana is ust like too much radioactivity - perhaps all mages could develop cancer early if too powerful...
To become a warrior in a tribe. -no food, drink, or sleep for 4 days and nights. -change into special clothes and painted enter hut -slices of skin carved from their chest and shoulders -wooden skewers through the bleeding flesh behind the chest muscles -stout thongs, secured to rafters were tied to skewers -hoisted from floor by these and weights were attached to their feet -twirled around till fell unconscious -when recovered from this, given a hatchet to cut off their little finger -ropes then tied to wrists and force to run in circle like a horse until he passed out -if survive all this he can return to his family in honor knowing he is now a warrior.