It has been established that 'Man of Steel' is the beginning of a whole new DC (Cinematic) Universe. Thus, we should anticipate some characters peeking through the door. Which characters were secretly referenced in 'Man of Steel' and has a possibility of appearing in subsequent movies?

Lex Luthor

Most casual viewers who have watched the Justice League animated series won't miss the various Lexcorp signs. On top of a building and on an oil tanker, it is Lexcorp.

This means that Lex Luthor is a part of this universe and at this point he is already established.

There was also a Wayne Enterprises logo on a satellite, but it is already a given that  Batman is in the sequel.

Supergirl

Now this one is more challenging. As Kal-el/Clark Kent was exploring the Kryptonian scout ship (which is like their version of The Fortress of Solitude), there was one pod open. That means there is another Kryptonian on the loose.

In an interview with Deborah Snyder (Zack's wife) she teased about that scene. The backstory to this can be read in the Man of Steel prequel comics. The reference is to Kara Zor-El who comic fans know as Supergirl.

Snyder's statements from Crave Online: "You know what's interesting, and I don't know and I'm not going to say what it means or anything, but the one thing if you look closely when Henry's going through the Fortress of Solitude there's an empty... you know all the bodies... the pods? There's an empty pod. I'm not going to say what, or if, it means anything but there is an empty pod there."

The direct question, is it Supergirl? "No. [Laughs] I'm not going to say anything but, it's a thing."

Booster Gold

Booster Gold is a time traveling hero who was formerly a member of the Justice League. Gold is a rich guy who wanted to be famous, and Blaze Comics was his pseudo-chronicle when he's playing super hero. Blaze Comics logo was briefly seen, but it hardly assures a Booster Gold appearance.

Since he's from the future, Booster Gold could have gadgets a la Iron Man, and since he's supposed to be a movie star/rich guy, eye candy casting potential.