I agree that a ribbon UI would be nice. For someone who works a lot with Office 2010, and other applications using the ribbon as well, it's simply amazing how fast you can access functionality that otherwise you'd have to find by navigating through endless menus, and you'd never find the needed functionality, even though it was right there.
A ribbon UI doesn't make sense in many applications, such as a web browser, but it sure makes sense in a download manager. So, I vote for a ribbon UI.
Modern Streamline GUI, therefore 1st choice (Very lite), but with Ribbon...
Just look Autodesk (AutoCAD or Revit) and Copy & Paste...
(you can hide all ribbon menu, but expand with one click! Therefore you can have it all... GUI is "Very lite" (all screen for work), but easy to navigate through complex menu (with expand ribbon, and tabs))
God bless FDM & all develepers