What happens if the prosecutor fails to reveal certain information to the defense?

If the complaining witness was stabbed 25 times, the prosecution has the ER photos and medical records, and you were caught red-handed ("red-handed," get it? heh, heh, heh.)... the victim's wishes ain't very exculpatory, dude. You want a real simple answer which fits every conceivable set of circumstances? Not gonna happen.

"If a prosecutor fails to reveal (after a request for discovery has been submitted) the fact that a complaining witness wishes to drop charges, can a Brady issue be raised? "

That's not a Brady issue. First off, Brady is about evidence - a witnesses wish is not evidence. Depending on what other evidence a prosecutor has, that wish may prove utterly irrelevant. If the DA can go to trial w/o the testimony of that witness, then the witness's unwillingness is completely irrelevant.

Next the Brady Rule is about evidence that could exculpate a defendant. A witness's unwillingness to testify has nothing to do with the larger issue of a defendant's guilt.

A Brady issue arises when the prosecutor has the information - the prosecutor is not a custodian of a citizen witness, and that witness is capable of informing defense counsel of his intentions.

Lastly, even when that testimony is critical, a witness's preference is completely irrelevant in the absence of evidence that this preference means he won't cooperate. Your question sort of assumes that, but it's not unheard of for unwilling witnesses to end up testifying. If a witness does not cooperate, that unavailability could actually trigger exceptions to the hearsay rule allowing other witnesses (cops) to testify as to statements made by that uncooperative witness.

The Brady rule has nothing to do with this question.

As a matter of law? Depends on details.

1) Is he withholding exculpatory statements by the witness (which he is required to share on penalty possibly including disbarment and prosecution) or merely failing to mention the witness does not wish to pursue charges (which is irrelevant?)

2) Are he and the Judge Democrats? Then none of the rules apply.

Depending what those charges are, it may not matter what the witness wants. If the prosecutor has enough evidence, they will continue with the case. It isn't uncommon for witnesses and victims to try and pull charges, and luckily the law often goes on without them. Intimidation isn't going to work, and the case will still be tried.

The wishes of the witness are not relevant, so there is no need for the prosecution to reveal it to anyone.

It would be a mistrial.