I usually just say that the function and limitations of a wish are determined by the source. In this way the wish is tied to a character who probably isn't going to put up with any kind of bull shit,  it can be tailored to any power level and it comes with built in story. 

For example

• Leprechauns can craft magic items for you, in particular shoes but have a tendency to do minor pranks
• Demons can get you almost anything but will do so in a way that requires an act of depravity. For example helping you murder some one for money
• Genies as elementals are limited to manipulating their element and have their own agenda which may be trying to be freed or enslave you 
• Arcane spell casters like wizards and dragons are limited to  arcane spell effects 
• Luck blades and the deck of many things manipulate luck or fate  so your wish comes through via a series of unlikely events. It can't magically manifest something, it doesn't occur instantly and you may need to put yourself in the situation where its possible. 
• gods are limited to their domains and tend to act in minor or mysterious ways using blessings, signs and visions