Jesper Christensen has been comparatively outspoken about his criticism of his earlier involvement. And he returned (for a bit.)
So Waltz could in all likelihood return…if it fits the script, that is. And that by now has been through various iterations without anybody officially suggesting Waltz would pick up his role again.
My impression was Waltz wasn’t happy with his part and didn’t expect to return. And if he was asked he‘d probably only do it if it suited his own preferences, his own idea of Blofeld.
The crucial thing: would Fukunaga be interested in bringing this Blofeld back? And if so, to what ends - would that script tempt Waltz enough to return? I‘m sceptic about both calls. But we should learn about it soon anyway.