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I'd only advocate a character if it made total sense for them to be there. And no one mentioned so far has any reason to be in Smallville, or on the show. I hated the Flash episode simply because it was totally random.
If there had to be one or two, one would be a villain,,, and it would be bizarro 'clark'. Why? because all the foundation is there in the plot already. ie. The Luthor's and there knowledge about the 'magic blood', the Cloning of Emily, the guy who got brought back to life by Lionel (with the kidney failure). All the ground work is already laid for a Bizarro character, an evil ******ed Clark. It'd make a good arc. Two clark's walking around for one episode (double impact style), then in later episode(s) clone Clark begins to break down mentally and physically.
It'd make a great arc
The other would be GL, handled much like Superman TAS handled it. A fallen Lantern would crash in SV, and the ring would scoot around until it found a 'clark' like dork. Villain would show up to capture the ring, and Clark would help this new superhero find his sea legs, and together they'd take down the villain