Wishes are fun, but they’re tricky to adjudicate. You’ll use them more if you keep them simple: a wish can duplicate the effect of any spell or potion in the campaign. The heroes don’t have to have encountered the spell or potion. The lower the level, the bigger the area or duration—maybe even make it permanent.

• spell level 1 to 3: as written but permanent duration or 10x area of effect
• spell level 4 to 6: as written but 3x duration or area of effect
• spell level 7 or 8: as written but 2x duration or area of effect

Of course, popular wishes would be for money and magic items, and—at least in my campaigns—such things can’t be conjured by a magic.

• If a hero wishes for wealth, maybe the people of a barony are charmed into believing the character is the rightful baron and oust the current baron. But the ousted baron and his cronies still think he deserves to be baron and so become dangerous enemies.
• Or he or she gets a vision of where to find a treasure hoard… with trouble matching its value. But this treasure should be in addition to what the party would normally get from such an adventure. (For example: a dragon with a double-size hoard.)
• If the hero wishes for a magic item, he or she gets a vision of where to find it… with trouble matching its power. But the item should be in addition to what the party would normally get from such an adventure. If such an item doesn’t exist, the magic finds something similar that’s reasonable.