What would happen if you wished...

Kyubey would probably say that's simply not possible. He/she/it made it clear that not every wish was necessarily doable. Although in Madoka's case, with her multiple timelines converging to give her a mega powerup, she's the one person who probably could have made that wish. 'Course everybody else would still be screwed.

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"Pride is not the opposite of shame, but its source. True humility is the antidote to shame."

You'd keep your soul.

In a compact, safe form called the Soul Gem.

Gotta be hella specific to Kyuubey, yo.