Bruce:
He calls you Cupcake because your so sweet, and he loves sweets so he names your after it.
You call him Big guy, or sweetie because you loved being normal.
Bucky:
He calls you Doll, or Babe, because he's from the 40's and that's what they had said back then.
You call him Warrior because he would die for you, and he went into WWII
Clint:
He calls you Peta because your first mission you where untrained, and you almost died multiplied times.
You call him Katniss cause he's an archer like her.
Natasha:
She'd call you Giant because you where so much taller than she was.
You'd call her tiny, or Ninja because she was tiny, and she was so fast with her flexibility you would think she was a ninja.
Pietro:
He calls you Penguin because they're slow, yet fluffy and cuddly.
You call him Flash because he knows how much you love the Flash, and you love annoying him by your fan girl screams.
Steve:
He call you Doll, or Darling because he's from the 40's
You call him Cap, or Old man because he's a captain, and he's super old.
Thor:
He calls you Queen or Princess because he thinks your secretly royalty.
You call him thunderer, or King because he's the God of lighting, and he's the king of Asgard.
Tony:
He calls you Sweetheart, because you where a sweetheart. Babe. And Iron Woman, because he is Iron man, and you where his woman.
Wanda:
She'd call you Bunny, because you where jumpy, and she could use her powers to pull you out any situation.
You call her Babe or Magician because of her powers.