There isn't anything too major, but there is stuff on what's planned for some future updates (most of the unreleased weapons and stages in the file data of Splatoon 2 are new versions of previous ones)
So here's a data mine of Splatoon 2 which shows all the weapons and gear in the game. Although be warned that it also shows future weapons planned for release. They're mostly reskins of existing weapons but there is one all new weapon we haven't seen before so be warned if you don't want to be spoiled. They shouldn't be all the future weapons though. Just the ones they made before the game's release so they could have quick frequent updates in the first few months of release. Anyway they're all listed in a table here: http://leanny.github.io/splat2/en_files.html
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More Information from reddit:
https://www.reddit.com/r/splatoon/comments/6rvvhd/major_spoilers_datamined_information_reagrding/
Some information on upcoming maps:
- Incomplete files for revamped versions of Kelp Dome and Blackbelly Skatepark have been introduced in the 1.1.1 update
- There are additional maps named Line and Tunnel which are in development
- Placeholder images for maps revealed in the Direct named Kawa and Pillar are present. (thanks to the people at TCRF!)
- Nintendo is including testing versions of Shifty Station in the updates; however, for Splatfest the game downloads the real stage model and layout file separately. This explains why the existing files include debug and test objects.
- The developers are currently testing new Salmon Run stages with internal names of Shakehouse and Threepoints. New files for these maps have been introduced in the 1.1.1 update as well. These are the last of the 4 planned stages for Salmon Run as of version 1.0.0.
How about some upcoming weapons?
- Grizzco brand weapons are in the works. A Blaster, Charger, and Brella were found in the files.
- A new weapon type called the Squeezer is in development. Here is Sheldon's explanation text for the Squeezer and an alternate version of it.
- You can find more about upcoming weapons on the full Splatoon 2 weapons list. (Thank you Lean for providing this service!)
A quick word about Octolings:
- What was known as the player's gender from the original Splatoon has now been replaced by a parameter called PlayerType.
- Another parameter called PlayerType shares the same values as another parameter, ModelType.
- The possible values for these parameters are SquidGirl, SquidBoy, and Oct.
- Near those values, we have found an additonal value named OctBoy. This is based solely on information found in the Splatoon 2 executable; we do not have models, text, etc for boy Octolings at the moment.
- Additonally, since the values of PlayerType and ModelType are shared, we cannot confirm that the Octolings will be playable either without testing in-game (which isn't possible for any of us to do).
We can't wait for all this content to be released. However, the game seems to be somewhat rushed to completion for version 1.0.0. Nintendo left behind a lot of things even though they tried to clean up the files. Marooner's Bay isn't even finished in this version and its English name is a direct translation of the Japanese text (Don Brako Shipwreck). Nintendo are secretly introducing new files via the balance updates.
