I worked with a Cuban chap who put bananas and pineapple in his guac, though the Cubanos have a different name for it which I can't recall. It was really good.
My favorite extra ingredient: bacon (unless you insist on authenticity). Two strips per avocado; less if it's really thick. Cook it until it's just firm, not crisp, and chop it up. If nobody's looking I've been know to toss in a wee splash of the drippings too.
Dave