Styling / Design
The design of ispconfig3 seems very old school - the icons aren't retina optimised (and dont feel up-to-date), color scheme is pretty default etc. and the page isn't responsive.
As the code seems HTML5 and slick, I guess a big jump could be achieved without major effort. Would be great if the look and feel of ISPConfig matches its superb features.
Icons to SVG/Icomoon/font awesome. Change colors to match a more up to date color scheme (your current website?)
Second Step: Responsiveness.