Have you read the essay advice from Philip Guo? It's an excellent resource, and he addresses your question:
http://www.stanford.edu/~pgbovine/fellowship-tips.htm
"I would strongly suggest writing a proposal that's somehow based on work that you have already done, so that you can tie it in with your Previous Research essay (cross-references!!!). In other words, don't just make up some cool topic out of thin air. If you're not that excited about your current work and desperately want to switch to something completely different once you start your Ph.D., now is not the time to express your discontent. It's dangerous to propose a research project that's not related to what you've already done because it dampens your credibility. Show, don't tell."