Emma Frost
The telepathic mutant Emma Frost has been long-rumored as a Vanguard due to her diamond form, which allows her to become essentially indestructible. I could see her using this to resist any incoming damage for a brief period of time, allowing her to safely disrupt the enemy team without risking herself. Her psychic powers could also give her some decent crowd control abilities that let her do some damage as she protects herself and others.
