This cannot be answered without knowing your world, your characters, and your story.
My suggestion? Start with what you've got and see how it plays out as you write. Part of the point of NaNoWriMo is to "just write it", to not let yourself get hung up on prep work or the perfect outline or any other hangups. It's a rough draft - its job is to be rough, so later drafts can be smooth. If you don't like your on-the-fly ending, it can always be changed in Draft 2, but sometimes you just gotta walk the path with the characters to find out where they go, and that means writing the words.
But if you're dead-set on having to know the ending now, my guess is something's still mushy in your world and/or characters. How well do you know your setting, and what possibilities that setting offers? How do your characters fit into (or fail to fit in, or attempt to change and fail/succeed) this world? What are their options? What sort of theme or feel are you going for? What strikes you as most "true" to your characters and your plot arc?
Good luck!