One look at Joyce Carol Oates's bibliography and you'll notice one thing:
She'll write about anything.
Blonde: A novel about Marilyn Monroe
My Sister, My Love: A novel inspired by JonBenet Ramsay
Zombie: A psychological novel from the POV of a serial killer
The Female of the Species: A collection of short stories about women behaving badly (very badly)
Beasts: A novella about a crazy university student
We Were the Mulvaneys: Oprah pick, family drama
And that's just some of her fiction. A teeny, tiny sampling.
What can we learn from this?
1. Write. A Lot. A great big heap of a lot.
2. Write about whatever you want, because if you're a good writer, you can write anything.
I realize that the last point sounds an awful lot like Randy Jackson on American Idol, but he has a point. If you can sing, you should be able to sing anything. That includes the telephone book. Singers sing. Writers write.
Oates doesn't limit herself to what topics she'll tackle. She doesn't say "Oh, I'm a YA, Romance, or Literary Author" and stick to just one kind of story. She mixes up genres and form. Three-book long family saga? Done. Horror novella? Done. Teen romance short story? Done. Biographical novel? Done. And let's not get into her essays and poems--which are extensive as well.
As writers, our only limit on subject should be our interests. (The skills to pull off a certain form are different story--that stuff takes practice. Like singing--you don't start off with that high C, you put a ton and half of hours into getting the control to get up there.)
If we've learned to tell a good, compelling story, then that will apply to whatever we write. So if you want to jump from an alien abduction love story to a Civil War ghost story, you are fully allowed to do that.