Hmm, that's a good question. I suppose if the game still has its core element, then it can be stretched out pretty far. Taking Mario Sunshine into consideration as an example, it was quite different from 64, with the FLUDD thing and an emphasis on cleaning. However, there was still that core element of platforming, and the FLUDD enhanced it with hovering, high jumping, and that... high speed running technique. Quite different, but still a core Mario game.
I suppose you can say it has to be fun as well. Mario's gotten out of his element with RPGs, and those still worked, also still having that core element of making Mario still.... Mario! It had those Mario staples, and was fun!
I suppose something like making Zelda a puzzle game like Tetris wouldn't work as it's too far. But yeah... as long as it still has the core thing of said IP and is enjoyable.
Dance my pretties!
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