The EF 200-400 f/4L IS 1.4x with the built-in 1.4 teleconverter is not the only lens planned to have the technology. It will definately be the first, and may be the only one for a while, but the concept is being tested with other zooms as well as prime lenses.
...A 70mm~210mm f/2.0 w/ built-in matched 1.4X ...
I'd sell a liver for such a lens. (You can get by on just one.)
You would have to, based on the price of the PRIME 200mm f/2, and the fact it would be the first f/2 zoom ever with an image circle for a 35mm format.
I believe the rumor is the possibility of a 120-300mm f/2.8 1.4x TC (to give 168-420mm f/4 with the TC engaged). The overall dimensions of the lens might not be very different from the 70-210mm you stated (especially with regards to objective lens size), but the pricing for the two and internal design would be very different. Either way, this theoretical lens would be a baby Siglauncher (aka. Sigzilla, 200-500mm f/2.
But now that I think of it, wouldn't the 120-300mm (168-420mm f/4) conflict with the 200-400mm f/4L IS 1.4x USM? Is there a point (for Canon) to produce both? Wouldn't the 120-300mm have to be cheaper to justify its existence? I know it's just a rumor, I'm just thinking out loud.