A less interesting idea(that can make you some money) - Hack a Wordpress theme using
http://underscores.me/ and Twitter Bootstrap. I did it as a weekend project last sat/sun.
Another idea :
Knowledge Explorer : Write a scraper that crawls sites like HN/Reddit/StackOverflow/Quora etc and organizes articles/questions/discussions etc in categories like "Python", "Ruby", "UX", "Machine Learning". Allow users to explore it according to these categories and sort it on basis of votes/comments/answers/etc. It will involve web scraping and machine learning. I need it personally.