A few quick ideas
A few plug ins I'd like to see.
- A Metadata plug in for Shelfari. I know they don't have an API so it'd have to be done via scraping which isn't ideal for users and can be an outright PITA for the developer if the site changes. But it's a very good source of well maintained, and very comprehensive metadata and cover images that I find myself using more and more often to verify and correct metadata I get from other places. So a metadata plug in for Calibre using it would be excellent.
- A Sync/support plugin for Google's My Library. I switched over to using Google from Goodreads (for various reasons that aren't important). However it was reluctant due to the Goodreads Sync plug in's excellence. I would love to see a similar plugin for Google.