I'd suggest finding something that will be useful and is not currently available. Preferabaly something that is useful to you, but not necessarily. E.g. I recently decided to convert my sites to HTML5/CSS3. It was only when I came to validate a site that I found that the WDG HTML validator is not HTML5 compatible. I used that validator because, unlike the W3G one, it can validate a whole site in one go. Given that you need to validate the CSS and links as well, this makes validating an HTML5/CSS3 site quite cumbersome.
I considered trying to front-end the W3G HTML validator to make it process a whole site but decided that there was a problem with concatenating the validation output from each page, so I decided to streamline the process by:
a) Enabling the validation of the HTML, CSS and links from a single input.
b) Enabling the use of pre-written lists from which to sequentially select the pages to validate.
Initially I created my own structure for the file lists but then I realized the similarity to the sitemaps lists, which can be used to enhance the crawling of your site by search bots. So I rewrote them to conform to the sitemaps standards in both text and XML formats.
Since then I happened to get interested in the sentencing standards for UK magistrates and Crown Court judges. They are quite complex, and it seems to me would benefit from a web page per offence group with a calculator to indicate the recommended sentence range according to the various factors. It's actually turning out to be more complex than I realized. So whether I will have the time to complete it I'm not sure, particularly as I'm working on it "on spec", with no certainty of any return.
Anyway, I'd suggest finding a project like that, if you can...