I believe Virtualmin is the best control panel out there for managing websites. That is why I have already purchased 3 Virtualmin Pro licenses. I just wish you could improve Usermin’s read mail module. It is very powerful but Roundcube’s interface is more visually pleasing. Could you please consider improving the interface or have a Roundcube plugin which will make Usermin’s spam reporting/deletion, email blacklisting and mail filtering available in Roundcube.
That sounds like a pretty nifty request, I’m know that the guys at Virtualmin are always looking for ways to improve the product, and would agree that Roundcube’s interface is a bit more prettier for handling email.
I’d recommend that you also post a comment in the Roundcube forums, because Virtualmin makes use of standard Linux principals and someone might be able to build a plugin (or may already have) which may further assist you in your goals.