Talking to Italian movie site Talky Movie, actor Christoph Waltz answered the question of his return to Bond 25 with the following:
"No, I'm sorry. I'm really sad, but that's the tradition, that there is a new...name. Sorry. I would've liked to."
The tradition he refers to is a rather incidental one, in that Blofeld has been played by a new actor in every screen appearance since 1967's You Only Live Twice.
So Waltz seems to be saying Blofeld will be back in Bond 25, but with a new actor in the role. He could just be parsing that information from the fact that he's not been asked back, or he could just be a LYING LIAR, since he repeatedly denied playing Blofeld in the months leading up to the release of Spectre.
Waltz was not remotely the problem with Spectre, so I'm as sad as he is if he's been recast. On the other hand, I will get a perverse thrill out of the thumb in the eye a recast would be to the continuity obsessed.
But I'll state here again that there's an exciting and timely villain already embedded in curent Bond continuity. Make it happen, Eon. #PresidentGreggBeam