The bookies favourite is still the ED30/Cupra engine at present.
I read somwhere once not so long ago that the EA888 had things that prevented VW using it as the basis of big power units. No baffles in the sump was one, can't remember the other things.
As the ED35 is just the mk6 swansong model I reckon the scale of production will mean a tried and tested unit will be used. Alternatively you could guess that it could be the forerunner of the mk7 GTI engine using a modified EA888.
My money would be going where the bookies put theirs. We'll know for sure fairly soon.