Does this even work? Has it already been done?

It can work as Terry Pratchett has shown in some of his Discworld novels.
Mort - Death's apprentice (in the novel MORT)
Susan Sto Heli - Death's grand daughter (taking over his job in the novel HOGFATHER)

Every story has been done before. But your take on a story has not. Don't let that stop you.

Last year I had an idea but no plot. I know that according to Chris not having a plot is no problem but it was for me. I ended up writing about my characters but they never went anywhere. I kept waiting for the plot to show up, but it never did.

Thank the Nano gods I have a plot this year. If I was you I would go with the story that has a plot. If neither of them has a plot. Spend time in "What if" land.

Here's a twist....What if this story was like "The Apprentice" There are several people inline for this "job" Your main character is the only one who does not want the job. She does everything she can to get fired. But for some reason she always gets the job done. The other candidates hate her for it and try to sabotage her. In the end she realizes that her compassion is what makes her the best one. The others enjoy the job. Their goal would be destroy everyone.

Or. LOL>... Donald Trump is the Thanos character and your MC finds that he is totally past it. It is her job to get him to retire and prevent him making mistakes.

Or What if on her first assignment she can't take someone life . She let's him live, only to find out later that he is a serial killer. To her horror she realizes that because she had compassion on him. He is causing innocents to be killed before their time. Because he killed someone to take his place the serial killer will not be taken for a year. Now she has to keep him from killing until the next "window is open for him."

What if the MC's first job as the angel of death is her boyfriend?

You get the idea.

The first draft of anything is shit. ~~~~Ernest Hemingway

It is better to write a bad first draft than to write no first draft at all.
~~Will Shetterly

I think it sounds interesting. I spent so much time last year developing the back story for a couple of my characters that it took over and became the more interesting part of the book. There's potential there for plot. Death is a fascinating character - what a job.
What is Death's history? Why does he need an apprentice? Has he done this before? How long do apprentices last? Is there a new Death every few generations? What was the girl's life like before she was taken? How does she feel about assisting Death? Does she take people from pain? Where do all the other dead people go if they're not Death's new assistant?

If the girl is going to be his assistant forever you need an end point for the story. If not, then then you have an ending, which will help. Meandering without a plot or a destination will get you lost, but if you know where the story ends meandering your way there could be a lot of fun.

What's your other idea?

I can't honestly decide whether or not it would fall under the category of mainstream fiction or fantasy. I would be leaning towards the latter, but it would totally depend on how in-depth to the death stuff you actually go.

Last year's story for me was a goddess who lives a normal human life. I had a lot of difficulty calling it fantasy, because it's dealing with "human problems" but there were a couple of gods who lived as humans.

I finally fell into the general label "fantasy" but if I'm given to parsing it down, I call it "urban myth."

Hope that helps!

***
05 Nano: (finished as a lurker)
06 Nano: "And She Was" - winner
07 Script Frenzy: "Complicated Shadows" - winner
07 Nano: "ASW 3.0" - winner

Stories that involve death can be humorous (the new TV show, Reaper), serious (Meet Joe Black, Death Takes a Holiday), erotic (The Devil in Miss Jones) or any number of things. Don't worry about where your novel fits. Just write it.

Oh, and a death character story sounds like it could be incredibly fun to write. There are so many different ideas about death in every culture that you have lots of fodder for novel fuel. I'd run with this idea; but make sure you have an ending in mind so you don't meander. (I meandered and the book sucked. Now I make sure I have a beginning and an ending at the very least.)

Use your imagination and have fun.

2005: What the World Needs Now (romance) WINNER
2006: A Dish Best Served Cold (literary erotica) WINNER
2007: It's the End of the World As We Know It (literary fiction) WINNER
Born on the 12th of NaNo (yes, I have updated my age)

Makana, is it bad that your ideas sound so fun it makes me want to ditch my on novel? The one I've been excited about since September and have really dove into planning lately?

I'm interested to know what *your* plot is like!

*bumbles off to Makana's page*

Stubborn, I'd definitely say and of Makana's ideas make your story more than doable.

No story has ever been done just ONCE. Just be creative with an original idea and it will make the whole storyline superb! Take a spin off something if you have to like in the novel Mort. It's OK to repeat ideas-as long as you don't copy the whole plot!

Don't cry because it's OVER, smile because it HAPPENED.
~Bianka Rose

lol I love the "Apprentice" idea, and the idea that someone says "WTF? Death is really bad at his job! I have to convince him to retire!" Both sound very fun.

If the art of acting is reacting, then the art of writing is re-writing.