I'd like to have seen George Lazenby remain as James Bond in Diamonds are Forever and through the Moore era. He could have played a vengeful 007 hunting down Blofeld in DAF and would have been still relatively young had he remained in the role until the mid-1980s.
Robin Hood: Prince of Thieves with an actor who could sound English. Maybe Mel Gibson, if no actual Brit was available?
Kurt Russell instead of Sam J. Jones in Flash Gordon.
Alec or Adam Baldwin or maybe Michael Biehn as Batman (1989) opposite Michael Keaton's Joker. Directed by Ridley Scott or James Cameron.
A Bryan Singer-directed X-Men 3.
Richard Donner-directed Superman 2-4.