Another idea I had for a plugin yesterday - a "Manage Series" plugin.
With the future of the LibraryThing metadata download plugin questionable at best (and frequently mish-mashed quality of data even when it was working) the onus is going to be more and more on users to edit the series metadata themselves manually. I'm not a fan of the careful  bracket editing required in Calibre currently when you renumber something - it is fine for an ad-hoc edit but anyone who has tried to renumber a large series knows it quickly becomes painful.
My idea is a right-click option which will present the books you have selected in a list, allowing you to move them up/down to change their order in the "series". You would be able to specify a series name (defaulted from first found in the selection if any) and a starting number (defaulting at 1). It would display read-only columns of title, author, pubdate and the "current" series value, plus a new series value based on order in the list as you move stuff around.
Perhaps also supporting some left/right buttons for when you want an item to have a "x.1" number in the middle of the main sequence. Maybe also a button to bring up the FantasticFiction webpage for the author for those not using the "Search the Internet" plugin or who opened this dialog first...
This might also be useful to quickly create a "fake" series for an author where you want to read in order of "published date" but find the data sourced into Calibre is incorrect (as I do) due to the wrong ISBNs being picked up or data missing.
If it sounds of interest let me know.