I don't THINK this was suggested yet, but I'm not sure.
I would like a plugin to automatically download metadata in the background whenever a book is added without any.
Multiple sizes in the size column would be nice, too (or multiple size columns); then if there's duplicates their sizes can be compared to the correct size.
Not a plugin: Also, I've noticed that calibre has generally gotten a bit slower as I update lately. Just wondering if anyone else has noticed this.