I don't have a problem with a player trying something like that. The text of wish already refers to mechanical effects: for example "You undo a single recent event by forcing a reroll of any roll made within the last round (including your last turn)."
The fun part is how they actually manage to express that desire with an in-game statement. In any case, you still get to decide how to fulfill their wish, and how completely.
The barbarian expresses a wish to be 'faster in combat and able to land more blows than he is normally capable of'... I give him a haste effect!