as for amitabh in dhoom 3 😀
they would have to remove any kind of action and make it an intelligent thriller. these kind of movies does not work in india.
Amitabh as villian/negative for D:3 would be great, but as you said it would no more be action thriller, and turn into movie like Aankhen.
Better YRF should rope in some young actors like Shahid, Ranbir or Imran. Although it would be difficult to turn Shahid/Ranbir's innocent faces into negative shade. Imran could be perfect.
Or try some not-so-young Fardeen or Bobby Deol (anyways acting is not needed) 🙂