Loki will be a playable character in Marvel Ultimate Alliance 3: The Black Order, Nintendo announced during the Marvel Games Panel at San Diego Comic-Con 2019. Additionally, both Cyclops and Colossus from X-Men will be added via a free update on August 30.
The June-announced Expansion Pass will include three character packs for X-Men, the Fantastic Four, and Marvel Knights, respectively. During the panel, Nintendo confirmed that the Marvel Knights pack will include Blade, Moon Knight, Punisher, and Morbius.
Marvel Ultimate Alliance 3: The Black Order is due out for Switch on July 19.
Thanks, Game Informer and @AgentM.