I don't know who they had for him to begin with, but I think Sean Bean, who played Boromir, was an awesome actor and should have (and definitely could have) played Aragorn. He was far more kingly, even as Boromir, than Viggo. Extremely noble...and an excellent actor.
Personally, I think that Mortensen played Aragorn rather poorly, and I'm sorry that they had to botch one of the most awesome character in the book. (Sorry, Ainaserkewen.)