In my opinion, I would say no because it was born from a legendary (Whom shouldn't breed, anyway) and Legendarys aren't able to hatch from eggs (I know it is said Arceus was born from a egg, but it wasn't made by any other Pokemon) so Phione is pretty much to me Not Legendary material. Plus it doesn't seem to HAVE a power of a legendary, so it shouldn't really remain a legendary (PLUS it is obtained quite easily). If it were to stay a legendary, maybe they should change it a little so it can't be born from an egg or be Offspring-related to Manaphy, and then It'll be Legendary Material/A better legendary. Also they might need to add a bit more power to it because one of my earlier reasons stated above in this same post; Phione doesn't have the power of a legendary. I'm guessing moveset is already quite good (Since legendarys are good at moveset) and that it would be quite strong anyway. Other than that, it would have many advantages and disadvantages as a Legendary, who, was born by a Manaphy.