I do think it's somewhat overpowered to let the player choose from all trap types... but I like the idea of blessed wands of trap creation giving the player a choice. Same with monster creation (wand of wonder is completely random for a reason, of course).
So here's an idea for a way to make it just that little bit better, without making it too overpowered: give a prompt where the player can choose between two traps/monsters, chosen at random from the available options. The keystrokes required to choose could simply be up and down, or left and right, or whatever other pair of letters people think are appropriate. So it would go like this: "Do you wish to make a...", and then provide two options. A casting is used up by the time you get to this prompt, so cancelling wastes the casting (alternatively, you're forced to choose one of the two options, no cancelling).
By having two options with a blessed wand, you get just that little bit more control, but you can't just choose to make a teleport trap or blink dog on your first cast (unless you're lucky).
A similar thing could perhaps apply with the wand of ball lightning - if it's blessed, you can select a direction, but it only skews the probabilities (say, 30% chance of casting in chosen direction (eg/ L), 20% chance each in the two adjacent directions (UL,DL), 10% chance each in perpendicular directions (U,D), and 5% in directions adjacent to opposite direction (UR,DR), with no chance of it firing in opposite direction (R)), rather than turning it into a regular wand of lightning.