Are all wishes actually possible?
It could be assumed that every wish is possible, yet Kyubey may lie about a wish being possible since granting a wish will also drain the universe of energy. Kyubey will most likely only grant a wish if the energy outputted from a Magical Girl becoming a Witch is greater than what was needed to grant the wish, indicated when Kyubey sometimes says something like:


  Your wish has conquered entropy


It's assumed that the amount of energy to resurrect someone would be far greater than what would be generated by most Magical Girl/Witch transformations. After all, since Kyubey's race can manipulate souls (ripping them out of bodies and making them into gems), they probably know how much energy would be needed to make one.

The only wish which may in fact be impossible is something that affects the laws of thermodynamics in a way that entropy no longer exists. Otherwise, a Magical Girl like Homura or Oriko who knew about the transformation process probably would have sought out a Magical Girl who could make such a wish. (I'm assuming Oriko knew since she knew of Madoka becoming a witch who would destroy the world and would question Kyubey as to why.)

Of course, to change the laws of thermodynamics, it probably would need someone on Madoka's level to be able to produce enough energy by changing into a witch so strong it would destroy the universe.