I just finished writing up the Wish spell for my take on ODnD and Swords and Wizardry. The Wish spell has been a "problem" for DnD editions since it first appeared. And the number of ways of dealing have been varied.
My take is that unless it is granted by a malevolent being, like a demon, I will generally not try to screw the player over on the wish. However I do keep in mind it is just a 9th level spell so its power (or the ring version) is not infinite. In general I judge consequences of wishes on how well they fit in with the reality of the Majestic Wilderlands. If it looks like coincidence to an observer, then it will be granted without fear of side effects. More improbable result to the outright impossible results in "bad" things happening to the person making the wish. For me, my preference is to whisk them to an alternate reality where their wish is seemingly granted. Taking them out of the campaign for a long time if not permanently.
So after thirty years the time as come for me to somehow take what I do and write it up as a rule.
Magic-User, 9th Level, Range: Unlimited, Duration: See below
The Wish spell enables the caster to control the fundamental nature of reality. Unfortunately the more drastic the change the greater change of a backlash occurring that plunges the caster into an alternative plane of existence.
The safest use of a Wish is to cast any known spell except for another Wish. There is no chance of a backlash with this use of a wish. Also the caster can wish for unknown spells with effects similar to the ones known. For example a spell that acts just like a fireball except it is an explosion of ice and cold. Likewise there is no chance of a backlash with this use of a wish.
A wish can be used to alter reality. Using a wish in this way can result in a backlash. There are three broad categories a referee can use as a basis for deciding the odds of a backlash.
• Coincidental, if the result of a wish can be explained as a lucky coincidence by an observer, the caster will only suffer a backlash by rolling a 1 on a d20.
• Improbable, if the result of a wish would be viewed as highly improbable by an observer, the caster will suffer a backlash by rolling a 10 or less on a d20.
• Impossible, if the result would be viewed as impossible by an observer, the backlash is automatic.
• Outside of an effect similar to an existing spell, Charm Person, etc. Any wish effecting the free will of a sentient individual results in an automatic backlash.
The referee can assume that the hypothetical observer knows about the various uses of magic spells. The categories are deliberately made broad due to the flexibility of the spell.
When a backlash occur, the caster is sucked up into an alternate plane of existence. If the caster makes his saving throw at -5, he will find his way back in 4d5 years.
If the wish is successful the caster is mentally drained and is unable to cast spells for 1d4 days.