Legendaries shouldn't have evolutions save for Phione.
Reason: They're "legends", in a sense, gods. I would say all baring Manaphy are gods of the Pokemon world, deities that watch over the world.
Also, to be serious to the thread, I think that there should be a pre-evo of Kangaskhan. I mean, that little baby in the pouch. And having that could add one more twist: Baby plays with skull, it becomes Cubone: Compare Kangaskhan's child and Cubone
It might be hard to see with the small sprites (Too lazy to find large official art of them :<), but the similarities are there.