The creativity of DnD fans has been a draw to the genre for decades of course, long before the Worlds of Warcraft and digital downloads, good art has always been the backdrop for good systems and I am reminded of equivalent examples in Comic lore. To capture the imagination at a glance is worth more than a hundred pages of a players handbook to a novice and there are cultural icons that inevitably find their way into systems.
The most notable one is probably the Vampire which has claimed entire systems such as Masquerade as well as system extensions such as Ravenloff. Werewolves come a close second in my mind and, again, whilst having a formal monster presence in D&D, they are a staple of other horror systems such as Cthulu and Chill; lycanthropy is very often found on disease/curse lists. I do note as an aside that my spell checker wants to change lycanthropy into philanthropy... does that mean if you get bitten by Bill Gates, you become really generous on a full moon ?
Arguably Witches have inspired a lot of content in RPGs tho I am not actually aware of any systems built around them exclusively, which would make for an interesting Kickstarater, although they have inspired a lot of NPC generation and do in fact get very challenging to deal with in certain situations.
But as we are both cutting edge and topical at BRPS, I would like to present you to a 3.5e prestige character class of the Pumpkin King, lovingly created in the home brew section of D&DWiki.
and in fact its been taken on into 5e using artwork by Phil Berry at enworld.org