Gotta think the movie is likely a good 4-5 years away, so people need to be thinking ages.

People who are already in their early 30s wont be able to play Johnny when they're pushing 40, and to avoid awkwardness (and conform to Hollywood rules about female leads), Reed is unlikely to be played by someone in their 50s when Sue will will be late 20s/early 30s max.

Dont think about who would be perfect NOW - think who will be perfect by then. Sadly, a lot of great choices in this thread are going to have missed the boat because of age.