D&D is not a morality play or story and the rules that apply to wishes must restrict the DM as much as the player. The DM cannot be allowed to use his or her discretion. Everyone using a wish should have a clear and reasonable idea of its effect and limitations, enabling them to employ this magic without fear of arbitrary consequence.
Because of the difficulty of this requirement, and the very wide latitude of possible effects that a wish might have, there are consequences that must be considered regarding both the campaign and upon the enjoyment of the game. As Bernard Suits describes it, "Playing a game is the voluntary attempt to overcome necessary obstacles." We don't want to make the power of wishing so formidable that it will overwhelm the game and make every desire possible for the players, so that the game ceases to be worth playing. At the same time, we do not want to contain the effectiveness of the wish by means of threat, guilt, supposed punishment for choosing the wrong things or by means of the DM's autocratic authority.
The rules below are intended to solve this quandary.
Rule 1: the DM is not allowed to misconstrue the player's intended purpose. When making a wish, the player should be able to specify clearly what is wanted, in circumstances where what's asked for is not merely a thing or an event. Most of the time, the point of inconsistency will likely result from an interpretation of the rules on this page. The onus falls upon the DM to empower the player to make themselves clear, discussing the matter before the wish is made, if need be. The DM should also be prepared to ret-con the wish and its results in the time immediately after the wish has taken place, if need be, in order to ensure that the player is completely happy with their intended purpose. Take note: if the player takes an inordinate amount of time to settle on the desired wish in a situation where time is relevant, such as during combat, the player's indecision should be held against the player by requiring a round or more to pass before the wish's asking has actually come to pass.
In all cases, the DM should always provide the most likely standard thing, in the most standard fashion that is recognized. That is, a "wagon" will always be of the most probable size of wagon that is for sale at a particular town, and not something much larger or much smaller. If an earthquake is wished for, the DM should decide upon the most probable size of earthquake that would meaningfully get the attention of people (that is, "feels" like an earthquake). The DM should not produce some minor tremor and argue that it is "technically" an earthquake, or even that "most earthquakes" are only tremors. When we think "earthquake," we have an image of widespread damage. The DM should recognize that wishes are meant to be powerful and adhere to that standard. The player will be wholly justified in receiving exactly what was wished for, without niggling or poor practice on the DM's part.
Rule 2: the Player is not permitted to needlessly accessorize the wish. It should be understand that one wish means to ask for one thing. Desirably, the player should be able to wish for a thing or event, and then apply one adjective to that wish. For example, the player could ask for a blue wagon, but not a big, blue wagon. Thus the player is enjoined to minimally describe the effect or thing desired, without positing that it has additional abilities, features, persons to service the thing or any other addition which must be considered a an additional wish. With regards to an earthquake, a player could ask for a "big" earthquake or a "devastating" earthquake, but not an earthquake that alsospecifically targets a specific house. That's two wishes and cannot therefore be managed with one wish. If the word "and" or "but" or "also" can be inserted into a given wish, it is probably two or more wishes and should be exempted.
Rule 3: the Player's character cannot affect things the Player cannot fully comprehend or identify. For example, a player's character cannot wish to "abolish France." In no way can the player fully comprehend the full nature of France, or any other entity that is vastly beyond the player's conception. Likewise, the player could not wish to change the course of a river they have never seen and cannot comprehend (particularly in a world without clear maps), nor ask for the deaths of thousands of people who cannot be personally known and identified by the player, nor ask for anything on a scale with which the player's character has no reckoning.
The Player's character can wish for things that affect persons or entities about which the player has sage knowledge, but only insofar as that knowledge reaches. For example, a player who is aware of a King's existence on the throne of a particular country can wish for the death of that king (but cannot name the manner of death, as that would be a second wish) ~ but would not be able to wish for the death of a specific King's advisor whose name isn't known, though it is fairly certain the King does have an advisor. Take note that to ask for the death of the most trusted advisor would break rule #2; the death of a single advisor could be asked for, but not specified.
In general, it should be assumed that the player's character can wish for things that directly affect his or her self; places where they are or have been; creatures which they recognize or can name and events regarding the consequences of which can be reasonably comprehended. The fallout from the obliteration of France is far too uncertain to be included in this list ~ and we must assume that the combined will of the gods and all other magical beings in the known universe are there to check the ad hoc disappearance of massively important entities.
Rule 4: The Player's character cannot meaningfully alter the game's fundamental structure. Players will be inclined to wish for the most obvious things: to be stronger, to be smarter, to possess more experience, to be able to cast more spells and be harder to hit and so on. Since this rule must take into account the running of an 18th level mage able to cast a wish spell [placeholder] every day, the possibility of increasing every stat of every party member, and all their followers, to a maximum degree, is a game breaking circumstance.
Therefore, it is argued that while the Player may be able to conceive the difference between, say, having 10,000 x.p. and 20,000 x.p., the player's character cannot. If the character were to ask to be wiser, more powerful, etcetera, they would be doing this without the benefit of having numbers assigned to these things that would tell them they were more powerful. Therefore, while the character would believe they were more experienced, wiser, stronger and so on, the character's actual stats would not change.
The character's wish might be more specific. If the character said, "I wish to be strong enough to break open that door," that would be granted ~ but it would NOT translate into a change of stats and it would not translate to every door. If the character were to say, "I wish to be strong enough to break every door," the specificity of the wish is lacking; it requires an answer to the question "where;" the implied answer, "in the universe," is an accessorization of "every door," and therefore is counted as two wishes. Not just every door, but every door somewhere.
Likewise, characters cannot be made smarter about things such as sage abilities. They can wish for an ability such as, "I wish I could ride a horse," which can be translated into the minimum of sage ability that enables this. A character could not, however, propose, "I wish I were an alchemist," because the player has no conception of alchemy and this would break rule #3.
ConclusionFor the present, I am convinced these rules serve to contain the power of the wish, while enabling a considerable amount of latitude on the player's part. In general, I believe that a player possessing a wish should be entitled to wreak considerable effect on the world, within the above boundaries.
It should be noted by players that wishes do not occur in a vacuum. If a king were wished dead, very likely the source of the death would be known within minutes, as the king is surrounded by clerics and mages with divination spells and other determination methods at their disposal; the king would likely be wished alive again within the hour, which would then create a focused effort to identify (again, through various means), the name of the original wisher, the location of said wisher and, within a few hours, the presence of a number of high level officials and potentially assassins teleported to that location for the purpose of arresting, killing or otherwise punishing the wrong-doer.
Players should always be aware that the presence of wishes in a campaign are a sort of nuclear option, where everyone with the power tends to treat its effects upon the powerful with a no-hands-on policy. Mess with them and they will mess with you. And since everyone in the world agrees to this cold war, there are many, many more ready to stamp down on a non-conformist than the number of non-conformists the player character likely knows.
No doubt, more theoretical points will have to be added to this page, given future instances where the wish crops up in a campaign. In no manner should it be assumed that this page pretends to cover all possible contingencies.