Well you would still be able to ease people into a Legion show. I'm not saying that a Legion show would be gigantic. I think it would be able to sustain the same ratings that Smallville gets each week, Alexz Johnson is by no means a house hold name but she did carry "Instant Star" here in Canada on her shoulders for four seasons before it eventually ran its course. As Saturn Girl she was amazing. I think if you had Geoff Johns and Paul Levitz as Executive Producers and a sound show runner team in charge they would be able to make it work.
I think viewers would be interested in seeing the FUTURE of the DCU...and that this would be the linchpin of the series. Immortal beings like Ra's al Ghul (who actually was involved in a plot to become the United Planets President and take over), Vandal Savage, and of course their own rouges gallery would be a focus. Plus the every day troubles and tribulations of a teenager would be character arc focus. I'm almost totally convinced a Legion show would work.
Not to mention the fact as I stated before in another thread Clark's history with the Legion is well documented.