Strong cheekbones, starry eyes , and other angular facial features complement star design motif of the character. Her dark hair also can easily blend in with a starry field. An older actress with a ageless/goddess air about her, who conveys strength and authority.
Whatever happened to Catherine Zeta-Jones? I don't know, but she's my nomination to play Varda. Catherine Zeta-Jones is a beautiful actress, but at 46 years old we would be steering clear of the 'trophy-wife' trope. (Props to her husband Michael Douglas, by the way.) She also has the long dark hair wanted to mirror her relationship with the night sky and strikingly lovely eyes. And her dark hair and light skin creates an interesting parallel with Melkor which we can play with visually.
Catherine Zeta-Jones also happens to be Welsh, so come Sindarin time, she'll fit (speak?) right in.