Anyway, I can't think of anything that would prevent a Smallville fic from being considered as An Early Years L & C fic - no canon reasons, given our wide open definition of canon, and our exclusion of character traits from it. Can anyone?
I think it's the character traits themselves. There are a lot of traits that stick with LCTAOS which aren't necessarily found in the other incarnations, and some that can't be found in the other incarnations. Present to me three fics based on non-canon versions of each show, and I can probably tell you which incarnation they belong to. Such as every time I've seen Smallville (which, I admit, is relatively rare), there is absolutely no way I can see that Clark Kent as the Superman we know and love. And there are so many differences between L&C's Clark Kent and the movieverse's Clark Kent. Don't get me started on that, though, because I never did like the movieverse's version and the Silver Age. The character just didn't make sense to me.
Even with elseworlds and alt-universes, there are so many bits and pieces of the characters that are intrinsic to L&C. And I don't think there is a way I could find them in any other version.
