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...
An insidious creature, most likely somehow "related" to trappers and lurkers, the Dead Leaves (for no other name exists as of yet for this foul thing), hibernates for three of the four year's seasons, deep underground. Its active time is Autumn, when trees shed their leaves, depositing colorful carpets across the ground. The terror then emerges and blends in with the surrounding leaves, perfectly camouflaged, waiting patiently for unsuspecting victims. In appearance it resembles nothing more than a ten foot square, six inch thick, layer of bright yellow, orange, and red leaves. The only hint that someone is walking on top of it, comes in the form of an unusual amplified sound of leaves crunching underfoot. Too late usually, the victims notice this additional "crunch". The Dead Leaves will then swirl and "rise" up to smother and suffocate the victim, like a colorful, malevolent, boa constrictor.
Fire, as can be imagined, is particularly effective against this creature, but one has to *know* it's there before putting it to the torch. And there's the rub. The creature is impossible to "identify" in a large patch of fallen leaves by eyesight alone.