Will Plasma 5 eventually be as configurable as KDE4? Some of the things I miss in particular is the 'Date & Time' Long and Short formats, Plastique Widget style and Plastik Window Decoration.

Plastique was provided by Qt (not a KDE feature at all) and eventually dropped in favor of "fusion", the plastik window decoration is still available.
Time/Date format is a matter of it's own, mostly caused by relying on Qt here now - see https://bugs.kde.org/show_bug.cgi?id=340982