I think it would be better to have standalone Black Panther and Spiderman movies beforehand.
Black Widow and Hawkeye in the first Avengers were also off, and they were also the only ones without movies.
You need to show us the characters first before making a crossover dammit!
I was afraid of this, but it ended up not being a problem for me. The characters were developed well in a very small amount of time.
Hawkeye didn’t need an origin story, he’s just an average soldier that uses a bow instead of a gun.
Black Widow could have used an origin story though.