You don't have to look very far to find references to the Man of Steel in Gotham — Arkham Knight is bursting with Superman easter eggs.
Helpfully, Arekkz Gaming has rounded a few of them up in this video:
Some of the more easily-spotted ones are the Lex Luthor corp ads populating some of the skyscrapers in Gotham:
Here's another Lex Luthor building spotted by CabooseXBL:
The second reference outlined in the video is a message you have waiting for you at your penthouse. Apparently, Lex Luthor is really interested in conducting business with Bruce Wayne — but Bruce isn't answering his calls! Luthor takes offence to that, of course. You'll have to watch the video to listen to it — it's great. Start the vid at the 1:06 mark to see it in action.
And the third reference found in this video is another poster, which happens to advertise Superman's stomping grounds:
Other players are finding Superman easter eggs, too. Redditor Thisty for example has spotted some ambient dialogue that points in that direction:
Robot Rock911 finds that Gotham thugs are pretty fond of Metropolis, actually:
The enemies actually seem more scared of Superman than Batman!
Picture: MathChief - Gameplay & Trailers!
There very well may be other Superman references hidden inside of Arkham Knight, but these are the ones people have found so far. The speculation right now is that all these winks and nods are actually 'proof' that Rocksteady's next game will involve Superman somehow — be that in a Justice League title, or something focusing entirely on Superman. It doesn't help that Arkham Knight has a handful of Justice League easter eggs, and that Rocksteady has stated that this is the final Batman game for them. The thought on everyone's mind seems to be, "what's next?"
On the other hand, a recent Rocksteady interview has them saying that Superman is "almost the impossible game to make." Maybe that's misdirection. Or maybe people just have a lot of wishful thinking, and these are just fun easter eggs. Who knows!