6. Semantic and phraseological change
Note: to find a word, you might focus on words with a moderately high frequency in COCA. (Note: might use WordAndPhrase site, or the Word Frequency site, or just click through sample words from the first page of COCA.) Make sure it's one with a range of different meanings -- e.g. not something boring like academic or examine. Might also check WordNet to see how many "meanings" there are.