I too would love eye control focus - I'm upgrading my EOS 3 (the original - with ECF) to a 5D3 at the weekend, and will miss it a great deal.
But let's get real: the 5D3 does not have ECF, even though it is focussed on sport, wedding and events, where it would have real value, particularly if the spot metering was linked to it, as in the EOS3 classic.
The chances of a 3DX having it, when it is likely targeted at landscape and studio, are remote in my view.
And I reckon it will cost more than the 5D3 too...
But who knows. Stranger things have happened.