"In future Missions and Spec Ops, you may find your Agent facing off against the likes of Red Skull, Sauron, and Kang the Conquerer, to name a few."
Since we already know that Red Skull will be faced by Captain America, so I'll focus on the other two.
Sauron is a Dino-Savage Lander, so he might be faced by Ka-zar (Kevin Plunder) and have a Mini-Boss of Kraven the Hunter, as Ka-zar and Kraven have battled in the past, in Astonishing Tales Volume 1 #2. The confrontation might be in a Spec Op that introduces the Savage Land.
As for Kang the Conquerer, he might be faced in a Spec Op or a whole chapter which introduces the Young Avengers, with Iron Lad being the Team-Up for Kang.