The character began as the name might suggest, a knight from the medieval era, though with a little bit of twist - his sword, the Ebony Blade, was forged from a meteorite by the wizard Merlin, making him a little closer to a superhero than his fellow sirs. Following this, a second, more villainous version wielded the weapon until it finally ended up in the hands of the criminal's nephew, Dane Whitman, in the modern day.
Since then, Whitman has made frequent appearances on the Avengers' roster, making him a pretty classic member. So, with rumours indicating that the hero will play a part in Endgame, it would be wise to assume the character could go on to player a greater role in Phase 4.
If said rumblings are to be believed, then the character has definitely already been cast. However, in the event that he hasn't, then Vikings' Moe Dunford would be an excellent choice for the role.