When writing a novel, authors generally don't want the reader to know how things will end. This is especially true of mystery novels, but obviously applies to any creative story. (A few stories show the end, and then the main question is how that end was achieved. However, the principle of keeping the reader in the dark remains the same.)
For this reason, avoiding a predictable plot is a good thing. You don't want to start setting things up, and then have the reader say to himself: 'Yep, I know how this is going to end.' This question is about how you can avoid creating such a predictable plot.
Here's an example which I recently thought of: Assume I'm writing a fantasy novel which takes place on an isolated island in the middle of the ocean. An amnesia-stricken newcomer arrives in the only village on the island, and quickly learns that life there revolves around escaping the island. The only way to escape the island is by defeating the evil monster keeping everyone from leaving. However, no one has yet been able to slay the monster.
You might not know exactly how, but you can tell that the amnesia-stricken newcomer is going to be the one who kills the monster and frees the people. The story will probably even end with them sailing off into the sunset. Forgetting the cliches for the moment, the plot is easy to predict.
Question: How can I avoid creating such a plot? Or if I have a predictable plot, as in the example above, how can I fix it? Are there simple steps or methods I can follow?
Note: Obviously all readers expect the good guy to win and the conflict to be resolved. That goes without saying. This question goes beyond that, referring to the times when the reader can list things which he knows will happen by the end of the book. It's more than knowing that the good guy will win. It's knowing how he'll win.
One method I've seen used is to establish a predictable plot or plot point, and then do the opposite, only to turn back at the last second. You still end up where the reader expected though, so this doesn't really solve the problem. It simply arrives at the expected outcome through unexpected methods.
An example is The Hunger Games. In the beginning of the first book, we all expect Katniss to enter the Hunger Games. She does, but only after Prim is chosen instead of her. We weren't expecting that, but she still ends up where we knew she would.
As I said, I don't see this as really solving the problem.
Note: Not a duplicate of this question. That question refers more to genre conventions, while this question deals with plot, and keeping the ending hidden from the reader until the last moment.