Most of us know that you should never, ever try to use the wish spell to become immortal. For the rest of you…well…
1: A portal to a particularly imperialist section of the Far Realm opens, and the character is sucked in by something with tentacles. Their brain is extracted and used as a master copy for a new generation of thralls.
2: The character’s time stream is permanently altered such that a second of perceived time is ten years of real time. The character appears to be paralyzed, and cannot affect or be affected by their environment. This condition lasts until the end of the world.
3: The wish appears to have worked as intended, and the character is offered the chance to take on the campaign’s primary antagonist immediately. The antagonist utterly destroys them; the legend of the character’s folly persists for centuries.
4: The character is flung into the sky, where they become a celestial body. Further interaction with the world below is not possible until space travel is invented, though the appearance of a new star or planet may cause a stir.
5: The character’s body begins to decompose as if they had died; they retain full control until such time as their body falls apart completely. The character’s consciousness survives perpetually in the molecules of the body, which scatter as the body decomposes, and may become other things.
6: The character immediately reverts to infancy. They have no memory of their life prior to this point and must be reared anew. Whenever the character once again reaches the age at which the wish was cast, this process repeats.
7: Body and soul decouple. The body falls dead, but the soul becomes a ghost (with limited ability to attack or communicate as the DM sees fit). Any attempts to resurrect the body result in a random spirit inhabiting the body.
8: The character’s soul is sent to the Lower Planes, where it becomes a petitioner in eternal service to an archfiend (and may be given an impossible task to perform to earn its freedom).