You know, I can get behind this choice. Between Widowmaker and Symmetra, you have two types. One who used to be a hero but was corrupted into loving villainy, and who imagines themselves a hero but does wrong. Using a princess would be a bit much, so Rarity, particularly Nightmare Rarity, would make for a better Widowmaker than Symmetra, who thinking of it this way, would probably be Glimmer.