Who is saying those rumors? We haven't finished the screenplay, so there is no way that anyone could know that. Those are two extraordinary actors, so if there is space for them in the story, I would absolutely want them there. But I don't know yet what it's going to be.
Cary Fukunaga's response came during a Philippine Daily Inquirer profile. The answer to his question on the Blofeld rumor is Christoph Waltz himself. Last year, the Spectre star told an Italian movie news site that he would not be in Bond 25. It's not a lost-in-translation issue either, because you can see Waltz answer in English in a red carpet video.
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.
Granted, Christoph Waltz said that a year ago, and a lot has changed since then. Director Danny Boyle left Bond 25 as director, and Cary Fukunaga was hired to take over. It's not clear yet how many changes will be coming with that, but Fukunaga did just say that they weren't even finished with the new script.
Back in May, Ben Whishaw -- who has played Q since Skyfall -- told The Hollywood Reporter he was contracted to be in the next film, but that was as much as he knew. At that time, Danny Boyle was still attached. Since Cary Fukunaga and company are now reworking the script, it's not clear who will land where.
It's at least safe to say Daniel Craig will be back as James Bond, for what is expected to be his final 007 film after Casino Royale, Quantum of Solace, Skyfall, and Spectre. Other than that? Cary Fukunaga gave the Philippine news site more information on his vision:
In terms of what I can bring to change the character, Bond is on a character arc that started with Casino Royale, and I will be carrying that on. There will be changes, I am sure. As in any story, a character has to change in order [to have] a narrative. [...] Daniel is an incredible actor. Outside of Bond, I have been a fan for years of his work. For me, he was the best part of Road to Perdition.
Props to him for that Road to Perdition shout-out. I might make an argument for Paul Newman, but my arm could easily be twisted toward Daniel Craig. Anyway, Bond 25 is meant to start production in March 2019 for its current February 14, 2020 release date. It was initially scheduled to begin production right about now for an October 2019 release date, but that was pushed back after Danny Boyle left.