@uriuri: I dont maintain such a list, my philosophy is that you should work on something that interests you personally, that way, the chances of you sticking around to maintain it are higher
That said, look at the plugin requests thread, there are many feature requests there. See if something floats your boat. Or see the new features in the editor thread, that is full of people asking for stuff. I have implemented some of that stuff, you can look at my commits implementing that stuff to give you a good idea of how to begin.
Then IIRC there are one or two open pull requests on github which need to be cleaned up based on my feedback but the original authors disappeared, you can take them over if you like.
Heck you can just start a thread titled "coder willing to work on calibre" here and people will post their ideas for new features. If you like something, go for it, if it is a large feature, best to run it by me first, to make sure I am willing to merge it in principle.
@theducks: Both recipes and get books plugins are auto-updated to ensure they are the most current, regardless of calibre version.