My personal experience tells me that Superbands never work as expected (Asia, GTR, etc), but it's always fun to create your own monster:
Vocals: Peter Gabriel and Annie Haslam, perfect balance with two different and opposite voices that would cover all the musical ranges.
Guitar: Steve Howe and Steve Hackett, would also cover virtuoso playing with Hackett atmospheres.
Keyboards: Even though Rick Wakeman is the greatest genius in keyboards, his presence would guarantee that the band wouldn't last more than one or two albums
so I would go with Tony Banks, the perfect guy for band, his small ego (compared with Wakeman and Emerson) let's him be a part of the machine instead of the front man.
Bass: If we can choose dead people, I would go with Gary Thain, the greatest bassist in history (IMHO), but if you have to choose a living legend, I'd go with Tony Levin.
Drums: Even though there are great names to choose, I choose Manu Katche, he's incredible.
Flute and backing vocals: Who else, Ian Anderson.
Violin: Jean Luc Ponty, an underrated genius.
Iván