Hmm.
I really would want to see a Terraria type of fighting game. Imagine what you could do with it. As Terraria is a sandbox game, you could allow them to craft in the middle of battle like steve from smash. U could also add the bosses as playable character's after you beat them. There are even mods that you could add as content
like Calamity and thorium. That would make it a pretty good fighting game. I think...

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Doesn't mugen have those types of characters though?

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I don't know?

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hmm