I don't think the 5D3 will be split into two different models.
Splitting it up increases the risk of both models where consumers may not be satisfied with both and even if they are satisfied with one of the models, there can be a big loss financially on the second if it doesn't satisfy the market. It also creates confusion in market, which model would be suitable. Eg. Someone who primarily wants photo use but also needs good video usage, which one would they buy if the 5D series is split up.
Canon make their money on a model that repeatedly gets purchased time and time again over its life span. Eg. 5D2 has been one of the best selling cameras of all time. Canon have had this camera on the production line time and time again.
I think Canon should make their necessary changes such as MP increase, ST-E2 transmitter, AF focus points, Digic 5+, 3.2" clear view II screen, increased weather sealing etc.
With these changes, the 5D3 will sell itself.
The problem is though your potentially looking at a much more expensive camera with those changes, ok for the higher end of the market such as former 1Ds users but it wouldnt cater to the market the 5D mk2 selling at around $2000 has tapped into.
Spilting lines based on features might not be ideal but if Canon have say a high MP 3D with those specs in the $3500 range and a lower end 5D mk3 with say the 18 MP sensor and maybe improved AF for around $2000 then they potentially tap into both those markets. They could also open up a market for users upgrading though the lines which I'd guess at the moment is largely confined to people buying used 5D mk1's first.
I agree... with all the cropped bodies having the same sensor, it seems crazy to me for them to announce a xxxD (as seems to be the annual trend, although that might change), before a 70D or 7Dmkii. I would guess a lot of new bodies to stabalise the whole lineup in the next 12-18 months!
One possible direction they could take is introducing the new processor into the D650 next spring while keeping everything else the same then market it on high FPS.