The Thunderbolts have been a long-awaited MCU team. The quasi-villainous anti-hero/misfit gang is a staple of the Marvel Comics world, and many heroes have been a part of it at some time or another.
In Thunderbolts*, Bucky will prove key to branding this group of characters together. In that same scene, Valentina Allegra de Fontaine assigned Yelena to kill Clint Barton, a.k.a. Hawkeye ...