replacement for the version II of the 300, 400, 500, and 600 with built-in 1.4 convertors (like the 200-400). Of course the price would jump by $2,000 per lens (ouch!)
Man, you are optimistic on price. Anyway, as the 200mm f/2L IS and 800mm f/5.6L IS look like bargains now in the supertele line, I'd bet on both of those being replaced with all the modern improvements of the other version II lenses, a matching paint color (those last-generation lenses look so outdated with different paint color /sarcasm) and an 'updated' MSRP.
Also good possibilities (only in the 'big' news realm):
45mm TS-E L and
90mm TS-E L
300mm f/4L IS replacement and
400mm f/5.6L IS
Then again, who predicted that Canon would replace the 15mm Fisheye with an 8-15mm zoom? Maybe we are all thinking a bit too much inside the box.