The Japanese ARMS Twitter account has teased a new arena which is coming to the game soon.
Biff says (at least from what we can make out from the rather ropey Google translation) that the arena is based in some ruins which lie "deep in the forest". This battleground was apparently home to ARMS contests when the association was established many years ago.
We don't get to see much of the stage, but it looks pretty cool, with the ruins and overgrown plants giving it a different feel from the other arenas in the game.
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