The elephant in the room, the low-hanging curve-ball is this: Anything by ABBA ! 'The Way Old Friends Do','Arrival','Eagle', and 'I'm a Marionette' would be So Cool(among others...)! Where am I wrong,eh...?
How about some covers/reinterpretations of another Kajiura favourite, Mike Oldfield. I'd love to hear how Kajiura-sensei and the girls would interpret the last part of 'Ommadawn Part 1', with its mysterious lyrics that are (I believe to be) Kajiurago's ancestor, with Korenaga-san whipping his guitar to feverish pitch--it would be a worthy cover! Of Oldfield's more pop stuff, a lot of the songs Maggie O'Reilly sang for him would be good candidates--'Family Man'(his version is so superior to what Hall + Oates did with it),'To France','Talk About Your Life', or 'Five Miles Out'(Kalafina would do nothing short of making a good song incredible!). Then there is Oldfield's song 'Pictures in the Dark'--Kalafina would be cool, but I could also hear Itou-san contributing to that one,too(those who have a better understanding of operatic voice range are free to tell me why I'd be wrong--I just know what I like!)
And then, there's Depeche Mode: 'Pimpf' would be cool given the Kalafina treatment, but I'd be fascinated to hear them tackle any DM material--'Pipeline', 'Black Celebration', 'The Mercy In You', and 'Sister of Night', a DM song that could've been great if they'd just gone for it! In DM covers, you'd be looking at changing keys and such--Dave Gahan doesn't have the girl's range, after all--but I'd be intrigued to see how they'd interpret them--or maybe I'm just nuts....