I'm a HUGE fangirl and nerd. I imagine nowadays most every fiction-based TV show gets at least a little fandom. I'll just stick to the ones I know and love:
Since you said "think outside the box" I'll skip Star Trek and Star Wars!
Buffy the Vampire Slayer and/or Angel, also known as the Buffyverse. Generally people fall in love with and follow the BtVS creator, Joss Whedon, and then you have a vague "Whedonverse" fandom.
Firefly aka Browncoats (again, Whedon).
Avengers, specifically MCU - Marvel Cinematic Universe.
The Walking Dead (TV show more than the comics)
Doctor Who (and spin-offs), a classic of course though not as well-known outside of sci-fi circles as the above-mentioned Star Trek/Wars.
Fandoms that have pretty much died but hold many happy memories and still-around fanworks: Pushing Daisies, Reaper, Dead Like Me, Lost, and The 4400.
Game of Thrones and Once Upon a Time are big right now.
Oh, Sherlock (the BBC series), can't forget that.
You can't get much larger in present time than the Supernatural fandom.
Not many have heard of it, but The Dresden Files is gaining popularity and is a fantastic book series, so truly deserves a fandom.
Also, off the top of my head: The Hunger Games. Divergent. The X-Files. (General) DC fandom. (General) Marvel fandom. Teen Wolf. Vampire Diaries.
Fun! I love fandom so much!