There is already a Rubik's Cube map (a Donator map). When thinking of themes, think about what you're good at building.
If you're good at people and organics, build a character from a show or a video game.
If you're good at architecture, build a famous building or a recognizable location from a show or video game.
Nature and biome maps are pretty unoriginal at this point, but if you can think of something original and can build it well, go for it!