The Thermodynamics of Wishes...

Wishes, those reality bending rewards; they come in bottles and on rings of power.  But beware always the capricious and trigger-shy game master...

Some GMs are understandably fearful of the game-breaking power of wishes, and this frequently results in them becoming non-existent or guaranteed bummers no player wants to touch.  And that's a pity, because wishes add an element of wonder and the possibility of reversing unfortunate events.  Yes, much mischief can be done as well, perhaps leading to memorable adventures (in retrospect of course).  But if the players somehow manage to find a magic lamp (or whatever), it should at least potentially have value.

Wishes alter reality, but thermodynamics insists that nothing can be truly created or destroyed, only moved around; and herein lies a guide for managing wishes in a campaign where they're desperately needed.  We'll begin with categories:

RESTORATIVE wishes affect the user (or a chosen target) to restore or reverse some personal misfortune.  If Bjorn was somehow reincarnated as a badger, he should be able to wish himself back to his former self, and denying this or imposing penalties to avoid breaking the game probably looks and feels like meanness.  Of course, the player must still wish responsibly.  Asking to be human again is not the same as wishing to be restored to their former (read: previous) identity.  But even then, the GM can respect intent... 

ENHANCEMENT wishes improve appearance, abilities, etc.  This is a mixed bag because some desired enhancements are potentially more game-breaking than others.  When wishing to change gender and/or modify appearance, it's pretty much doing a hard reset of the character concept.  This can be done with few consequence beyond the campaign narrative and may be interesting.  On the other hand, wishing to improve abilities has more serious implications.  Maybe each wish only raises an attribute by one point, and upon reaching the racial maximum, only by 1/10th of a point.  AD&D took this approach.

Of course, wishing to fly or shoot lightning bolts out of your eyes can significantly alter the fabric of reality.  In this case, the added powers absolutely should come at a price.  I suggest transforming the aviation enthusiast into a sentient pigeon or striking the lightning-bolt lover blind as their new power (forever theirs) blasts the eyes out of their head...   

Now we've entered bummer territory.  Tread cautiously here you would-be wishers!  

EVENT REVERSAL wishes involve any reversal that extends beyond an individual to the entire party (or even larger groups).  This requires seismic shifts to the fabric of reality because it literally reverses events for several people.  If half the party died in a terrifying pit trap - poof! - they're restored.  But every subsequent benefit, including magic items and levels gained by any survivors, are lost because the events never really happened.

As a rule, restoration/reversal for individuals is fine.  Restoration for groups requested by survivors actually with that group results in the loss of any and all subsequent gains because that much reality can only be altered by reconfiguring the "mass" of prior events.

GRANDIOSE wishes involve any fabulous (and external) acquisitions that can potentially unravel the campaign.  These always involve a trade-off.  Ask for infinite riches and you'll be teleported (quite alone) into Tiamat's lair.  Wish for the Wand of Orcus and enjoy an all-expense paid trip to the demon lord's throne room in Hell.  Requests of this magnitude can only be realized with great effort, and taking the wisher to the goal is typically easier in a cosmic sense than bringing said riches to the waiting character on a silver platter.

Ultimately, the idea here is that (1) wishes involve a sort of conservation of energy, a thermodynamics of reality.  Nothing new is added, just moved around, and (2) smaller and more reasonable requests involve far less effort and should be granted in the spirit of compensation for treasure justly earned.  Do this, and the players will know to exercise due discretion, which rewards them and keeps the game intact.  We can only wish...