After legal disputes forced the character away from the franchise for over 30 years, he returned in 2015's Spectre, this time played by Waltz. The character was revealed as being the mastermind behind the titular global criminal organization, as well as James Bond's adoptive brother, so it was a real shame that after his survival at the end of the last mission, he wouldn't be coming back.
When Waltz realised he'd been rumbled, he reportedly said: "You haven't seen me".
However, UK film journo Baz Bamigboye, who has undoubtedly become the premiere scooper when it comes to James Bond movie news, reveals that Waltz was spotted filming on the untitled production's Pinewood Studios set, although the actor apparently attempted to keep it a secret. Well, true or not, it's certain that Waltz would probably return in a lesser capacity than last time, and surely, Bond 25 producers are keen enough to let a newer Oscar victor take center villainous stage here. No info has leaked out about Waltz's role in Bond 25 (which still doesn't have an official name as of writing), but you can't imagine he would be the focus of the action, especially with Rami Malek playing the villain of the piece.
This is likely a simple cameo designed to create another link between the James Bond movies, continuing the connection of narrative tissue that has helped make this iteration of 007 stand out from all of the others. It already had a hell of a time getting off the ground with the replacement of director Danny Boyle by Cary Fukunaga. The film's now in production, finally, although Daniel Craig was substantially injured at some point. We haven't finished the screenplay, so there is no way that anyone could know that. Things seem to be on track for now, so hopefully we'll hear more about this one soon.
Bond 25 (which I hope gets an official title soon so that I can stop calling it by that clumsy name) is now in production and is scheduled for release on 8 April 2020.