I would cast Chris Rock as Tidus, because his kind of edgy and provocative humour would really bring the role of Tidus to life.
As for Yuna, I think I would cast Dame Judi Dench. She was rather impressive as M in the Brosnan Bond films. Once she was captured by that naughty Renard chap and that woman...I forget her name but she had a disfigured ear...anyway, she knocked a stool which had a clock stood upon it over, then dragged the clock over and took the battery out of the clock, then used it to make a tracking beacon work. I think this is the kind of thing Yuna would do if she was captured by Renard and the woman with the disfigured ear, and so Dame Judi Dench would make an excellent Yuna.