I just saw the movie and went home to check whether the website www.rataalada.com actually exists or not, and surprisingly, it does! Judging by the news articles I found while searching, it's been made for promoting the movie. Let's see if we can decipher something from the website about the sequel.
Here is the screenshot of the text that comes before the page settles down:
I recognized 1939 in the first IP and confirmed that Batman first appeared in the 27th issue of Detective Comics (released in May 1939, i.e. 5/1939). I'm not sure what the corresponding text means here. Is that a reference too?
After this message, the screen has a [CLICK FOR REWARD] button followed by a LOADING whose percentage increases with time. When you click the button you get the following image, appropriately named, Its_Not_Over_Yet.jpg:
I'm not sure how to decipher this without a key, but maybe the background for the image could help.
Hoping to see some interesting answers for this one!