Cartographical Jukebox: Pick a song and make a map that is in some way inspired by that song. It doesn't have to be directly tied to it, but just something that reminds you of the song.
Parallelism: Map out a movement that happened IRL as happening earlier. An earlier Civil Rights movement or an earlier Russian Revolution, for examples.
Biggering: Depict a nation that has suddenly and surprisingly gone expansionist.
I particularly like the first one for its originality, but the other two are very good challenges as well.