I'm confused. I've found tons of Legendaries with their own unique appearance (Morphs which are different from my Hero weapons) but then I find lots who seem to have adopted their (My Hero weapons) Morphs...?
the only weapon that morphs dynamically is the one that you can name. if you rush through the story, use primarily quick and uncharched sword swings, and upgrade melee at the first possible chance every time, then your weapon will vastly resemble the legendary weapon called "The Splade". i joined several games and most people seemed to have gotten it.
my advice: try not upgrading your melee right away and do some exploring if you want a diverse weapon.