A lot of the cake toppers I make express a fandom of some sort. They range from very specific (like the Halo or Tardis cake toppers) to broader areas (like fantasy armor or weaponry).
Now, I don't know that I've ever met a fan who was only a fan of one thing. So a lot of the toppers that I make like to mix some elements from one or another sort of fantasy universe. Here are a couple cake toppers for couples that mixed it up that way: