I personally don't think that he should play Superman on the big screen. I've been a fan of Smallville for 10 years, and have watched every episode. Although he has had some very good performances... I still feel that there's something not quite there... something lacking. Of course, this could be because he hasn't ever played "Superman" (well apart from briefly in "Homecoming").... and has only played a young Clark Kent... but there's a certain quality that Christopher Reeve had, and even Brandon Routh had... that isn't quite there.
Another issue is that if he was to play Superman, and then if you had a different Lois (not Erica Durance), a different Lex (not Michael Rosenbaum)... a different Martha Kent (not Annette O'toole).. it'd just feel weird (IMO). Whereas if you did get all actors to fulfill those roles, you'd just have a Smallville/Superman TV Movie.
I think a clean slate is the best way to go. Hopefully a relatively unknown, although I wouldn't mind Brandon Routh again. I think he did a good job, considering the awful script