Note: Just because these cards are in the game files, does not necessarily mean that they will be obtainable.
Credits goes to the uploader and dataminer over at reddit and discord. Source
In early February, someone was able to datamine and extract the game files. It contains audio files of characters using certain cards, list of cards and icons. While they are in the files, this does not necessarily mean that they will ever be released. Credits goes to the uploader:
Someone uploaded the card images to imgur showing the SRs and URs. Again, just because it is in the game files, it does not mean that they will ever be released. Credits to uploader, link to the source at the top of SR imgur page.
Images showing new cards along with their rarity has started to spread around. These are only rumors with no confirmation if they are real. It is not yet known if they are cards for the next pack or possibly for the KC event.
More pictures and videos have surfaced showing Yami Bakura and Yami Marik. Getting a good glimpse of their dialogues, skills and cards. With the conclusion of Pegasus event, either of the two might come very soon for the next event!
Images have started spreading around showing Mokuba Kaiba, similar to the style used in the game's pictures with different emotions, a battle pose and character selection screen. Could he be the next new obtainable character!?
List of game icons includes unused ones
Source: Image uploaded to Imgur
The other image shows the game's icon list, which also contains information showing that there are XYZ and Synchro icons, a possible confirmation perhaps?. It is only a rumor with no announcement on how long it will come, if it even will be.