Speaking to Empire Online, Superman Returns stars Brandon Routh said the sequel will definitely up the action:
“It’s going to be a lot more in that genre. In the first one, the obstacle was Superman’s love for Lois and his son, and Luthor using that against him. I just know that in the next film there will be a lot of action and I’m gonna get to fight something, or someone. An enemy with real physical power might be worked in there, definitely.
And who might that something or someone be? While he doesn’t answer that question, he talks more about the villain and the pre-production here.
Source: Empire Online