I think a lot of fans (don't ask me why) have an obsession with choosing elf races who just sneak around (yawn) whereas the best/most entertaining character would surely be either a warrior Nord! They look cool too imo. Anyway, a film of Oblivion is something I hope is made one day. And as for the protagonist, you'd surely want someone human looking (which excludes the elves and beast races) as you would probably be able to identify with them more. That's why they should go with a Nord (though an Imperial or Redguard wouldn't be so bad, though Bretons are too wimpy to be taken seriously so they're not even worth considering imo).
Anyway, I'm surprised Bethesda hasn't gone down that route as I'm certain it would be as popular as LoTR and Harry Potter (in fact, Harry Potter is complete tosh and is only for under 12s and is very hard to take seriously. Dumbledoore and Harry look like cheap imitations of Gandalf and Frodo!).
Anyway, I think they should cast Patrick Stewart as his rightful role as Uriel Septim and then concentrate on the main character's initial escape from the prison. I think we can safely discard weak, pathetic elf (esp wood elf) races as the main character and instead give it to a Nord (the best race by far). I think an actor like Steven Segal (he still has charisma and stage prescence) would be a great male Nord lead. He is physically powerful and it would be great watching him escape from the jail and take on the deadra!
Making a movie of Oblivion should be high on Bethesda's agenda imo, and I'd gladly like them to focus on that instead of any other games they might have lined up. What about others?
EDIT:
My breakdown of characters:
Main Guy: Steven Segal
http://andaimedaarquitetura.com/andaimedaarquitetura/wp-content/uploads/2009/02/steven_seagal.jpg
Jauffre: Derek Jacobi
http://jimberkin.files.wordpress.com/2007/09/cadfael.jpg
Martin: Sean Bean
http://gracemagazine.files.wordpress.com/2008/01/sean-bean.jpg
Baurus: Samuel L Jackson
http://www.tvtango.com/uploads/blogImages/100/samuel_l_jackson.jpg
Uriel Septim: Patrick Stewart
http://img2.timeinc.net/ew/dynamic/imgs/080808/sesame-street/Patrick-Stewart_l.jpg