Create an idea that requires you to do several things for which you would like to learn. This idea should ideally include offline processing of work, so that y

Create an idea that requires you to do several things for which you would like to learn. This idea should ideally include offline processing of work, so that you are forced to read and understand many issues with executing jobs outside of the client/server request cycle (maybe this is image creation, video/audio transcoding, log file analysis etc etc). Then make sure your project can leverage some sort of key/value storage system like redis or memcached, or maybe a document store like mongodb or couchdb. Finally, make it something that people like so you'll get enough traffic to need zero downtime deployments. If you make a project like this, you will learn most of the high value systems/concepts being explored and demanded right now. Additionally, if you satisfy the last constraint, you may have a cool successful startup at the end of it.