As population increases, animal souls are promoted to human before they are spiritually advanced enough in terms of impulse control, etc.: this shouldn't have happened, but karmic progression is automatic, after all--it's a force of nature. But the 'animalization' of humankind bodes ill for civilization.
No doubt the same goes for elves, dwarves, and so on.
Magical curses are rarely cast, as if cast on someone for fun or purely maliciously and the person cursed has not done something seriously wrong, it tends to ricochet back on the curser. Therefore the few who do get cursed are shunned by most people as genuinely guilty,and few will help them.
Chekov's Spell -- This spell has a two-fold effect. The first is to make the target item almost-unnoticeable to anyone not specifically looking for it. The second is that someone who has seen the item will remember it when they encounter a situation that would be perfectly solved by use of said item.
The words of a wedding vow contain a strong love spell which enchants both the people who says it, so they will allways be deeply in love with each other.
Little halflings are told that if they don't behave, a demon called Santa Claus from a hell of ice will kidnap them and force them to toil making toys.
What if a invisible person could see other invisible creatures? All you would need to do to check for nasties hiding around is turn invisible.
Imagine that all the humanoid and demi-human laguages are actually the same, but pronounced with outrageous accents and bizarre idiom. All the elves have a French accent, all the Dwarves have Swedish, Dragons have a Pakistani accent...
Under the sewers of a large town ancient burial chambers are discovered. If the PCs investigate they fill find that a pale white flowers grows amongst the graves, in the dark.
How do they survive without light? What is their origin? Why in a burial chamber and not just in any old cave?
Can they be sold or do they have any special quality at all? It is up to the PCs to discover.
The air had grown chill the minute they descended into the strange valley, which was unmarked on any of their maps. It was so strange here, devoid of animal life and completely silent. The horses were nervous the entire journey through the vale. As they set about to collect firewood for the campfire they could hear their own voices as dim echoes through the eerie silence.
The food didn't taste anything that evening and their sleep was cold and troubled by nightmares. While they are clearing camp the next morning, one of them stumbles over a piece of stone jutting out of the ground nearby the horses. On closer inspection there seems to be runes engraved into the polished surface. The symbols true meaning is no more known among mere mortals and if they decide to dig deeper, they will discover that it is an ancient altar buried within the soil.
Any historically oriented party member will recognize the largest symbol to be the insignia of the powerful warlock who ruled this realm several centuries ago. At their departure from the area, something will seem amiss with one of the party members and all will remember the stories of the warlock's thousand curses.