Concerning Waltz´ return:
I think that BOND 25 just cannot omit Blofeld.
It is the end of the Craig era, which was tied together in SPECTRE as Blofeld´s long standing evil plan to hurt his stepbrother. To introduce Blofeld and Spectre in SPECTRE and then get them both out of the way in order to start fresh for the finale would be a huge waste of one of Bond´s biggest and most important enemy.
And since Blofeld therefore has to return, Waltz also should. I don´t think recasting the role would work or do it any favors. Waltz is a capable actor, and if he gets more screen time and more chances to shine he still can be the menacing, sick bastard who employs many henchmen to actually finish off Bond.
BOND 25 really could be a film which has the opportunity of having the backstory already explained in SPECTRE and now can go full throttle right from the start.
Blofeld has escaped/rbeen released from prison and kickstarts Spectre again. His main plan now can be to unleash terror on the world, knowing that this would bring Bond back on a collision course - but now Blofeld does not have to explain himself anymore, he just can go berserk on Bond, killing off Madeleine, maybe even M, and delight in the way Bond seems to be outmatched.
Really, this is the only way to go, I believe.
BOND 25 has to be a new version of YOLT.