Just some ideas you can have fun with.

The List:

-Steampunk
-Cyberpunk
-Anime themes (Pokemon, DBZ, SAO, etc))
-Country themes (England's mailboxes, Paris's Eiffel Tower, etc)
-Game items (Portal Gun, etc)
-Nerd stuff
-Hipster Lyfe
-Fossil theme (Dinosaur Age)
-Matrix (Computer Language)
-Alice in Wonderland
-Kitsune
-Holidays (Halloween, Thanksgiving, etc)
-Tropicals
-Dragon Age
-Military/Army
-Nightwear
-Paint ball
-Lazer Tagish
-Clockworks
-Ninja
-Dark/Gothic/Evil/Murderous/Killer
-Summery
-Technology things

I'll add to the list. I found my other list that I posted on a different thread to give ideas. ._.