When they first introduced the concept of Black and White allot of my friends, and me included had the hunch that they were going to introduce a new pokemon type e.g. Light (to go with the already existing dark type).
I think that if they added something like that it would have given gen 5 a little bit more substance.
As it is that didn't happen so saying they should be light / dark types would be kind of cheaky. What I might suggest would be fire/normal and electric/dark.
I also agree that instantly making the assumption that dragon=epic, legendary=epic therefore dragon=legendary is quite unimaginative.