When the empress hands Bastian the magical sand and asks what he will wish for, he obtusely stares at her and has no answer. He should instantly state the wish that he has had since before finding the book: for his mother to be alive. Bastian has unlimited wishes, and the sand has no restrictions on its power because it's from Fantasia. He can restore a land that no longer exists, so there is no reason why he can't do the same for his mother.
shareHis mother is already alive. Just not in a physical body at that moment - why do people still not realize that physical body is not anything but a dead vessel that only SEEMS alive because a soul is living in it..?
Also, Fantasia is the world of IMAGINATION, so he can't bring changes to the REAL WORLD, except purely physically. He can't wish for the Earth to explode (or he can WISH, but it won't come true), for example, but he can wish for Artax to be alive, because Artax isn't REAL.
What Bastian has is just a HOLODECK - you can create things that are 'very realistic', but you can't affect things outside the holodeck. He can't manipulate other people's lives, free will or bring people back to the physical world from the astral side, or he could look at any historical atrocities and just decide to bring people back that died in wars and whatnot.
His power does not manipulate the 'real world' (or he could turn IT into Fantasia), the power only creates 'very realistic figments of imagination'.
In effect, he could've brought a facsimile, something that looks, acts, behaves and sounds like his mother, but it would not have been her actual mother, because Fantasia is the world of imagination, and the only way to affect the real world is to bring things from it into the real world.
However, even that might be an illusion - it could be argued that the ending with the Luck Dragon and the bullies didn't REALLY happen in real life, Bastian simply imagined it happening, so it happened to him 'realistically' (again, think of Holodeck), but the actual bullies never saw any Luck Dragon, and never jumped into the garbage bin.
I mean, if he could have any wish he wants in the REAL world, why would that affect the world of imagination, and then couldn't he wish to live on a different planet, do time traveling, change the political system, end hunger and wars, and and and..??
Obviously, he couldn't, even the narrator says that he 'returned to the real world' (or something).