Powers given to a Wishborn upon their contractual agreement are in some way related to the wish that they have had granted. All powers are given to the player to decide for themselves, and given only the guidelines that their imagination is the upper limit of what these can be, so long as they are at least in some way or another related to the Wish they have made. Obvious restrictions exist, such as unpreferred and gameplay-hindering effects it may have on other players or the stability of Roleplay as a whole. If a player isn't sure of how a power they come up with could work, or if it would be accepted altogether, they are encouraged to seek out Lilly (the Head of RP on the Cleft) and discuss it with her; there have been very few suggestions that have been turned down so far, which means it is likely that most ideas are able to be managed somehow.
Powers that exist to a player (maybe as they are from some other magical variant) prior to becoming a Wishborn do not require a Wishborn transformation in order to be accessed; only those related to their wish have this requirement. However, most Wishborn will find that Witches in particular are not susceptible to powers or abilities outside of the Wishborn variety, stressing the requirement that a character embraces his or her granted abilities in order to maintain their own survival in these cases.
For ideas on powers that relate to a wish or to get an idea on how the relation works, please see some of the character profiles listed on the main Wishborn page. A few of them list their abilities at the bottom of their character profile.