These oneshots/headcannons whatever you wanna call them will be filled with spoilers I a must warning you now so that you won't get mad at me later. 

Spoilers will be from movies including:

Ironman

Hulk

Ironman 2

Captain America 1

Thor

Avengers

Ironman 3

Thor 2

Captain America 2

Guardians of the Galaxy 1

Avengers Age of ultron

Antman

Captain America 3 (civil war)

Doctor strange

Guardians 2

Spider-Man homecoming

Thor 3

Black panther

Avengers infinity war

Antman and wasp

Captain marvel

Avengers endgame

Spider-Man far from home.

And tv shows including:

Agent Carter, Seasons 1-2

Agents of Shield, Seasons 1-7

You have been warned.