Any KDE based application could in theory start using qtquick instead of qt C++. I don't know why you would take a C++ implementation and make it run on qtscript, though.
I easily see new applications written against kde5 using a lot of qtquick and js implementations. Especially with Ubuntu phone coming, there would be little to change between some gui designs between supporting the desktop and phone.
That said, Plasma and KWin will probably have the most changes, one from the mass conversion to QtQuick, the other for supporting Wayland.