What would cause this Wish?

[Ken Murikumo]

It works... No, it really does...

BUUUT, with all the power comes the necessary knowledge of an overdeity. The knowledge of every timeline, every possible event, and every possible outcome (including the understanding of the infinite multiple cosmos and entropy) rushes into the players mind, destroying it. They never said anything about gaining the will or vessel to take on that mantle. This can lead to a few outcomes:

-their mortal mind is broken but the aspect of the overdeity they become functions as a was mentioned in the other posts about deities. With their mortal mind lost, they have no desire to influence anything the PCs do or will do. Write up a new character.

-Their mind is destroyed leading to the most powerful mad-god in existence. Congrats on creating the new campaign boss. Write up a new character... And good luck!

-Same as above, but the newly formed mad-god acts at random. Any Deus-Ex-Machina that happens from now on (even in the future campaigns) is now simply explained as a mad overdeity doing weird things.

[noob]

It works... No, it really does...

BUUUT, with all the power comes the necessary knowledge of an overdeity. The knowledge of every timeline, every possible event, and every possible outcome (including the understanding of the infinite multiple cosmos and entropy) rushes into the players mind, destroying it. They never said anything about gaining the will or vessel to take on that mantle. This can lead to a few outcomes:

-their mortal mind is broken but the aspect of the overdeity they become functions as a was mentioned above. With their mortal mind lost, they have no desire to influence anything the PCs do or will do. Write up a new character.

-Their mind is destroyed leading to the most power mad-god in existence. Congrats on creating the new campaign boss. Write up a new character... And good luck!

-Same as above, but the newly formed mad-god acts at random. Any Deus-Ex-Machina that happens from now on (even in the future campaigns) is now simply explained as a mad overdeity doing weird things.

-as the knowledge comes to your mind you figure out the meaning of the "everything": the everything you knew was in fact an entire fictional universe centred on your team and by becoming an over-god you might stop being important and thus that your entire "omnipotent" life would have less significance than one more day as a PC
So you use your unlimited power to become a Player Character but you grab this occasion to do some changes if you want(The gameplay effect of that entire wish is just a casting of psychic reformation).

[Remuko]

As much as I love the pettiness of Microcosm - and I do - and as much as I appreciate the honesty of a DM saying, "No, nothing is powerful enough to grant this Wish" - and, again, I do - I think this answer is, narratively speaking, the best.

The wish, as written and as intended, is granted in its entirety. The PC has ascended to become an uber-overdeity. And in his first act, he maintains the stability of reality by pinching off a fraction of a fraction of his essence, plopping it back into a mortal frame at the moment of his apotheosis, and then shuffling off beyond the conceivable cosmos. This tiny mortal mind is granted the bliss of ignorance, the mercy of not knowing the truth of its creation - it simply believes that it is what the PC was a moment before. Or perhaps the PC is aware, if academically, that it now both is and is not a beyond-cosmic being. And if, in dreams or the moments between moments, the PC feels the briefest taste of power or awareness of the great beyond, these phantom sensations are easily dismissed as mere fabrications of the mind.

This. I love the idea Malphegor suggested and Red Fel speaks of here. Its brilliant.

[Madsamurai]

Depends on how you think Wish works. If it is by you petitioning a greater power, then the "it works, but..." approach is fine. My interpretation is that the spell takes the finite power available in a 9th level slot (lets call it 9 mega-sparkles) and attempts to grant your wish as best as it can. If the wish is friendly, then it would respond to the wish by giving you a +1 inherent bonus to your best stat. If it is hostile or uncaring it will microcosm you :D

[One Step Two]

Pretty much what Red Fel said. Let the player create a new Over-Deity for the setting, and his character is a minor avatar to maintain the continuity of his life. That's what I did when I talked my DM into letting my character become a deity through in-game shenanigans, my character got a minor mark to show something had happened, but no change in power in any way.

[RNightstalker]

PHENOMINAL COSMIC POWER!
...itty bitty living space.

This made my day!

[weckar]

First of all.
Mystra sees such a powerful spell, and goes "Well I died last time someone tried this".

She says no.

Ao also sees this and realizes that would horrifically upset the universe.

He says double no.

In the realms, maybe. Elsewhere? It's probably worse.

PHENOMINAL COSMIC POWER!
...itty bitty living space.

This is slated to be an appropriate reference for many generations to come.

[Bad Wolf]

An example from the Epic Level Handbook:

For instance, if a character wishes that he be forevermore rendered immune to all damage, the character’s wish is most easily fulfilled if he is instantly eradicated from the multiverse, thus ensuring that nothing can ever hurt him again, now or forever.

Personally, I'd say whatever overdeity is there already comes in and squishes him before he can try to do that.

[gooddragon1]

What if a player Wished for:

" I want to became the most powerful overdeity of all multiverses and reality, able to annihilate, recreate and reshape the existent at will".

"Roll 1d4."

A slip of paper appears that has the following printed on it in gold lettering:

"<Character Name> is now the most powerful overdeity of all multiverses and reality, able to annihilate, recreate and reshape the existent at will."

Along with a,
1- Sapphire
2- Emerald
3- Ruby
4- Diamond

Worth an apparently large amount (24,999 Gp).

===

A one size fits all response. Partial fulfillment. Very partial and somewhat twisted perhaps.

"Looks good on paper, not as good in practice."

If they point out that a diamond would have more utility (if they didn't get one), you may remind them that such a request would have been possible for the spell.

If the request is too long or something,

"<Character Name>'s request was fulfilled."

and the gemstone granted.

The asking to roll a d4 might cause a moment of fear. So you can assure them that it isn't bad if necessary.

[Mordaedil]

Personally, I'd say whatever overdeity is there already comes in and squishes him before he can try to do that.

Hm, would manifesting as a giant foot in the best of Monty Python do or would it just be a giant thumb, crushing the player as an ant while the rest of the party watches on in horror at what transpires?

[Harkness]

Reply with "Pun-Pun takes an attack of opportunity on you for spellcasting within his reach."

Brilliant.