Good path for them would to mature the series as they did with (and how I hate to use this reference) 90210.
Idea goes:
Starts off as Smallville
Then goes to a new collage years series: Metropolis
Then as the ratings die go the route of Star Trek and launch in to a line of movies:
Same cast, huge franchise, solid fan base. This way gives the kid some time to hit the gym and literally grow in to the part.
Only other route I see, scrap it all together and have the movies completely separate.
No Way should the cast from smallville! They are all 2 young and tom although very good in smallville doesnt have what it takes to be in the suit (at least not yet)! Chris Reeve is a tough act to follow. Personally if they dont think Dean Cain has what it takes then i vote for an unknown, bringing a big star in will just ruin the movie for the true superman fans. Clark Kent/Superman isnt the only important role! They just have to have a good director and not have any idiots involved or this movie will end up being a waste!
they wanted to have josh hartnett play the part but he turned it down because he did not want to get stuck in the sequels, so that shows they want a younger acter, and if they take smallville clark kent they could easily look older, they make him look younger by make-up. But he has not been asked to play the part so it is unlikely they want him cause if they did they would have asked him to begin with,