betrayal, redemption, forgiveness, destruction, rebirth, indifference, inadequacy, loss, gain. Lots of things.
For me, the best way and the most natural way to develop theme is to allow it to develop on its own. Write the first draft and see where you went with it. If you sit down and consciously put in symbols to represent themes and scenes that only support the theme without any other function, the story comes across as fake. Of course, you can sit down and say I'm going to write a story about loss, or grief, or whatever, but after that, wait and see where the story wants to go.
When I got my degree in Lit one of my professors said that the worst person to ask to analyze a work was the writer. Because what the writer intended is not always what comes across, and very often the strongest symbols and themes came about organically without the writer being totally aware of it.
I agree with that to an extent. I do think that what was intended is very often not what comes across, simply because the writer brings a different agenda than the reader, different influences, different perspectives. However, I do think that as a writer we can sit down after the first draft and consciously look for where we can support the theme better, where we might change some things or some characters to better illustrate a point. But I think it should always serve the story rather than the other way around.
Theme must support the narrative, not the other way around. If narrative supports theme it becomes a lecture that tries to clobber the reader over the head. And sometimes you might find that the theme you wanted to write about is not the theme that emerged. Don't fight it. Go with what you wrote. It'll be stronger than something that's forced to fit a preconceived model.
I should disclose that I dont outline, ever. If you like to outline, I still say dont force it. If you veer away from the outline, go with it. see where it takes you. you might be surprised at what your subconsious wants to say.