I think Middle Earth would do good, but will it be to the level of Star Wars? I don't think so. Mainly because kids under 13 probably won't have watched the movies. Star wars has cartoons and things like that for the younger kids. Again, both of these are more "guy" type films. I am a geeky tomboy girl (watched Stargate, huge football fan, role playing, computers, board games, played magic the gathering in college, etc) and I am not into Middle Earth stuff or Star Wars. I probably could get into Star Wars if I tried and probably will try some day, but tried with Middle Earth and it was boring too me.
To be like WWOHP and draw in huge number of people for many years to come it has to be something so mainstream that all ages and all genders like it and want to see it. Star Wars comes the closest, I will give you all that, but I just don't see Middle Earth.