Other Worldly Gate
Create a gate or doorway into an other worldly place. Something would represent the gate itself, other structures might maintain or defend it. Perhaps its a gate into the First World of Fey, perhaps they're the gates of Hell, or the Pearly Gates, perhaps its an entryway into a Para-Elemental Plane, or a hyper-space jump gate.
This idea is similar to this month's Locus map for ley lines, which inspired this one. However this is more direct and a means of travel between our realm and the next.
EDIT: adding Ghostman's thoughts - create both sides of the gate, the cartographer is free to show this however way they want - that's an interesting twist!
To make this challenge more interesting and unique, I'd add a rule that the map must show both sides of the gate - how exactly that is done would be up to the mapper.
BAh - I should have waited on my entry instead of jumping in at the last moment....oh well, I can always redo it better I guess.
Good idea though.