A feature I'd love to see is an ability to read RSS feeds within RoundCube.
In the folderpane on the left, you could have the favicon of the website the feed originates from, followed by it's title. When clicked on, the messages pane would populate with each downloaded message, where you could then read them as you would an email.
Note, when I say "downloaded," I mean for RoundCube to save each feed message (file? db?) as it obtains it for future reading. Many RSS feeds only display the 10 (for sake of picking a number) most recent feed items, so if none of them are archived within RoundCube and you wanted to read one from a while back, you'd be out of luck.
If this feature were implemented, I could say goodbye to my current email method and switch to RoundCube fulltime.