I wanted to point out that I think Kyubey can refuse certain wishes because the wishes are powered by emotion, the hope that goes into the wish and the potential for despair that comes out of becoming a witch. If someone tries to 'break' the system with a wish (like wishing for infinite wishes or wishing for god-like power), it is usually out of thought and calculation rather than out of emotion, and therefore, there is no energy to power such a wish. Madoka's wish worked because of the accumulated despair from multiple timelines and her conviction to end Homura's endless suffering. It also came with the cost of non-existence and it's questionable whether it actually made the world a better place or not.

I just wanted to throw that out there because this gives the GM the right to refuse certain characters who may try to play the wish-granting system.

That is all.